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Chapter 1: Nutrition in Plants
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Chemical Equations
- Modes of Nutrition:
- Autotrophic Nutrition: Organisms synthesise their own food from simple inorganic raw materials (CO₂, H₂O, minerals) using sunlight and chlorophyll. E.g., all green plants and blue-green algae.
- Heterotrophic Nutrition: Organisms depend directly or indirectly on autotrophs for food. E.g., animals, non-green plants, fungi.
- Photosynthesis:
The biochemical process by which green leaves prepare food using chlorophyll, sunlight, carbon dioxide, and water:
Carbon dioxide (6CO₂) + Water (6H₂O) ⎯⎯⎯[Sunlight / Chlorophyll]⎯⎯→ Glucose (C₆H₁₂O₆) + Oxygen (6O₂) - Specialised Modes of Nutrition in Plants:
- Parasitic Plants: Non-green plants that obtain readymade food from a host plant using specialised absorbing roots (haustoria). E.g., Cuscuta (Amarbel).
- Insectivorous Plants: Partially autotrophic plants growing in nitrogen-deficient bogs that trap and digest insects to obtain nitrogenous compounds. E.g., Pitcher plant (Nepenthes), Venus flytrap.
- Saprotrophic Nutrition: Organisms secrete digestive juices onto dead and decaying matter and absorb dissolved nutrients. E.g., Fungi, Mushrooms, bread mould (Rhizopus).
- Symbiosis: Mutually beneficial association between two organisms sharing shelter and nutrients. E.g., Lichens (Alga + Fungus), Rhizobium bacteria in root nodules of leguminous plants.
- Stomata: Tiny microscopic pores present predominantly on the underside of leaves, flanked by kidney-shaped guard cells, responsible for gaseous exchange (CO₂ and O₂) and transpiration.
NCERT Intext Questions & Scientific Enquiries
Page 1 — Intext Q1
Why do all living organisms require food?
2 Marks
✅ CBSE Standard Answer
All living organisms require food for the following fundamental life processes:
1. Energy Source: Food serves as fuel; through cellular respiration, nutrients are broken down to release ATP energy needed for locomotion, digestion, circulation, and maintenance.
2. Body Building & Growth: Proteins and minerals in food furnish structural raw materials for cell division, tissue differentiation, and overall physical growth.
3. Repair of Tissues: Worn-out, injured, or damaged cells are repaired and replaced using nutrients absorbed from food.
4. Immune Resistance: Vitamins and essential minerals protect the body against pathogens and infectious diseases.
2. Body Building & Growth: Proteins and minerals in food furnish structural raw materials for cell division, tissue differentiation, and overall physical growth.
3. Repair of Tissues: Worn-out, injured, or damaged cells are repaired and replaced using nutrients absorbed from food.
4. Immune Resistance: Vitamins and essential minerals protect the body against pathogens and infectious diseases.
CBSE Marking Scheme 2026-27
Energy for biological metabolic activities½ Mark
Growth, cell division and development½ Mark
Repair of damaged and worn-out cells½ Mark
Disease resistance and immune protection½ Mark
Page 2 — Intext Q2 (Paheli's Query)
Paheli wants to know why our body cannot make food from carbon dioxide, water and minerals like plants do.
2 Marks
✅ CBSE Standard Answer
Scientific Explanation:
1. Green plants possess a unique green pigment called chlorophyll enclosed inside specialized cellular organelles called chloroplasts.
2. Chlorophyll has the unique biochemical capability of trapping and absorbing solar radiation (photons) from sunlight to drive the endothermic synthesis of glucose from CO₂ and H₂O.
3. Human and animal cells completely lack chlorophyll and chloroplasts. Therefore, even though carbon dioxide, water, and sunlight are abundantly available in our environment, our cells cannot trap solar energy to synthesize organic food molecules. Consequently, humans must rely on heterotrophic nutrition.
1. Green plants possess a unique green pigment called chlorophyll enclosed inside specialized cellular organelles called chloroplasts.
2. Chlorophyll has the unique biochemical capability of trapping and absorbing solar radiation (photons) from sunlight to drive the endothermic synthesis of glucose from CO₂ and H₂O.
3. Human and animal cells completely lack chlorophyll and chloroplasts. Therefore, even though carbon dioxide, water, and sunlight are abundantly available in our environment, our cells cannot trap solar energy to synthesize organic food molecules. Consequently, humans must rely on heterotrophic nutrition.
CBSE Marking Scheme 2026-27
Presence of chlorophyll/chloroplasts in plants to trap solar energy1 Mark
Absence of chlorophyll in human/animal cells making photosynthesis impossible1 Mark
Page 2 — Intext Q3 (Boojho's Query)
Boojho wants to know how water and minerals absorbed by roots reach the leaves of a plant.
2 Marks
✅ CBSE Standard Answer
Mechanism of Water & Mineral Transport:
1. Water and dissolved soil minerals are absorbed by microscopic root hairs through osmosis and active transport.
2. Plants possess specialized vascular plumbing tissues called xylem.
3. Xylem vessels form a continuous, unbroken pipeline of microscopic hollow tubes running from the root tips, up through the stem, into the branches, and throughout the petiole and veins of every leaf.
4. Evaporation of water from leaf stomata creates a powerful upward physical pull called transpirational pull (suction pull), drawing water and dissolved minerals upwards against gravity right into the food-making cells (mesophyll) of the leaves.
1. Water and dissolved soil minerals are absorbed by microscopic root hairs through osmosis and active transport.
2. Plants possess specialized vascular plumbing tissues called xylem.
3. Xylem vessels form a continuous, unbroken pipeline of microscopic hollow tubes running from the root tips, up through the stem, into the branches, and throughout the petiole and veins of every leaf.
4. Evaporation of water from leaf stomata creates a powerful upward physical pull called transpirational pull (suction pull), drawing water and dissolved minerals upwards against gravity right into the food-making cells (mesophyll) of the leaves.
CBSE Marking Scheme 2026-27
Role of continuous xylem vessels running from roots to leaves1 Mark
Role of transpirational pull/suction pressure lifting water column1 Mark
Page 3 — Intext Q4 (Paheli's Query)
Paheli wants to know what is so special about the leaves that they can synthesise food but other parts of the plant cannot.
2 Marks
✅ CBSE Standard Answer
Structural Adaptations of Leaves:
1. Concentration of Chlorophyll: Leaves possess millions of specialized photosynthetic mesophyll cells containing abundant green pigment chlorophyll, which captures solar radiation.
2. Presence of Stomata: The leaf epidermis features thousands of microscopic pores called stomata flanked by guard cells, allowing atmospheric carbon dioxide (CO₂) to enter the interior leaf tissues efficiently.
3. Broad Flat Surface Area: Leaves are thin, broad, and laminar, maximizing light interception and minimizing internal diffusion distance.
Note: Non-green parts like roots or thick woody stems lack chlorophyll and cannot capture solar energy. (However, in desert plants like cactus, modified green fleshy stems take over photosynthesis).
1. Concentration of Chlorophyll: Leaves possess millions of specialized photosynthetic mesophyll cells containing abundant green pigment chlorophyll, which captures solar radiation.
2. Presence of Stomata: The leaf epidermis features thousands of microscopic pores called stomata flanked by guard cells, allowing atmospheric carbon dioxide (CO₂) to enter the interior leaf tissues efficiently.
3. Broad Flat Surface Area: Leaves are thin, broad, and laminar, maximizing light interception and minimizing internal diffusion distance.
Note: Non-green parts like roots or thick woody stems lack chlorophyll and cannot capture solar energy. (However, in desert plants like cactus, modified green fleshy stems take over photosynthesis).
CBSE Marking Scheme 2026-27
Abundance of chlorophyll-rich chloroplasts in leaf mesophyll1 Mark
Stomata for CO₂ intake and broad laminar surface area1 Mark
Page 3 — Intext Q5 (Boojho's Query)
Boojho has seen some plants with deep red, violet or brown leaves. He wants to know whether these leaves also carry out photosynthesis.
2 Marks
✅ CBSE Standard Answer
Answer: Yes, leaves with deep red, violet, or brown colors DO carry out photosynthesis.
Scientific Justification:
1. Chlorophyll is present in these colorful leaves in normal quantities.
2. However, these leaves also produce large quantities of other non-green pigments (such as anthocyanins, carotenoids, and xanthophylls) which impart deep red, purple, or dark brown hues.
3. These intense dark pigments physically mask (hide) the green color of chlorophyll from human eyes.
4. The underlying chlorophyll continues to trap solar energy, and photosynthesis proceeds normally. This can be verified experimentally by boiling the leaf in alcohol to extract the masking pigments and performing the iodine starch test, which turns blue-black.
1. Chlorophyll is present in these colorful leaves in normal quantities.
2. However, these leaves also produce large quantities of other non-green pigments (such as anthocyanins, carotenoids, and xanthophylls) which impart deep red, purple, or dark brown hues.
3. These intense dark pigments physically mask (hide) the green color of chlorophyll from human eyes.
4. The underlying chlorophyll continues to trap solar energy, and photosynthesis proceeds normally. This can be verified experimentally by boiling the leaf in alcohol to extract the masking pigments and performing the iodine starch test, which turns blue-black.
CBSE Marking Scheme 2026-27
Affirmative confirmation (Yes, photosynthesis occurs)½ Mark
Explanation that other pigments (anthocyanins/carotenoids) mask green chlorophyll1 Mark
Verification via iodine test producing blue-black starch reaction½ Mark
Page 5 — Intext Q6 (Paheli's Query)
Paheli wants to know whether mosquitoes, bed bugs, lice and leeches that suck our blood are also parasites.
2 Marks
✅ CBSE Standard Answer
Answer: Yes, bed bugs, lice, and leeches are ectoparasites, and female mosquitoes behave as temporary parasites.
Biological Classification:
1. Parasite Definition: An organism that lives on or inside another living organism (the host) and derives nutrition directly from the host, usually causing harm or discomfort.
2. Head Lice & Bed Bugs: Live directly on the host's body/habitat, feeding on blood meals for survival and reproduction (obligate ectoparasites).
3. Leeches: Attach to the host's skin using suckers, secrete hirudin (anticoagulant), and feed on blood.
4. Female Mosquitoes: Siphon blood protein from warm-blooded hosts to nourish developing eggs (often classified as temporary micropredators or hematophagous parasites).
1. Parasite Definition: An organism that lives on or inside another living organism (the host) and derives nutrition directly from the host, usually causing harm or discomfort.
2. Head Lice & Bed Bugs: Live directly on the host's body/habitat, feeding on blood meals for survival and reproduction (obligate ectoparasites).
3. Leeches: Attach to the host's skin using suckers, secrete hirudin (anticoagulant), and feed on blood.
4. Female Mosquitoes: Siphon blood protein from warm-blooded hosts to nourish developing eggs (often classified as temporary micropredators or hematophagous parasites).
CBSE Marking Scheme 2026-27
Definition of parasites deriving nutrition from a living host1 Mark
Classification of lice/bedbugs/leeches as blood-sucking ectoparasites1 Mark
Page 6 — Intext Q7 (Boojho's Query)
Boojho wants to know how fungi appear suddenly during the rainy season on bread, leather shoes, and rotting wood.
2 Marks
✅ CBSE Standard Answer
Scientific Explanation:
1. Microscopic Spores in Air: Fungal spores are extremely light and are naturally floating suspended in the atmosphere around us at all times in a dormant state.
2. Protective Spore Coat: Each spore is enclosed in a tough, hard protective coat that allows it to survive long dry spells and unfavorable temperatures.
3. Monsoon Warmth & Moisture: During the rainy season, the ambient air becomes highly humid and warm. When these microscopic airborne spores land on wet, organic substrates (moist bread, damp leather, decaying wood), the moisture triggers instant germination.
4. Rapid Mycelial Growth: The spores sprout hyphae, secrete digestive enzymes onto the substrate, and rapidly multiply into visible cottony fungal colonies within 48 to 72 hours.
1. Microscopic Spores in Air: Fungal spores are extremely light and are naturally floating suspended in the atmosphere around us at all times in a dormant state.
2. Protective Spore Coat: Each spore is enclosed in a tough, hard protective coat that allows it to survive long dry spells and unfavorable temperatures.
3. Monsoon Warmth & Moisture: During the rainy season, the ambient air becomes highly humid and warm. When these microscopic airborne spores land on wet, organic substrates (moist bread, damp leather, decaying wood), the moisture triggers instant germination.
4. Rapid Mycelial Growth: The spores sprout hyphae, secrete digestive enzymes onto the substrate, and rapidly multiply into visible cottony fungal colonies within 48 to 72 hours.
CBSE Marking Scheme 2026-27
Ubiquitous presence of dormant microscopic airborne fungal spores1 Mark
High humidity and warmth in rainy season triggering rapid germination1 Mark
NCERT Textbook Exercise Questions
Q1
Why do organisms take food?
2 Marks
✅ CBSE Standard Answer
All living organisms require food for the following essential life functions:
1. Energy Provision: Food acts as fuel, releasing energy through cellular respiration to perform all vital life activities (locomotion, digestion, circulation, reproduction).
2. Growth and Development: Nutrients in food (proteins, minerals) provide building materials for cellular division, tissue formation, and body growth.
3. Repair and Maintenance: Damaged body tissues, worn-out cells, and injured organs require nutrients for cellular repair and regeneration.
4. Protection Against Diseases: Vitamins and minerals strengthen the immune system and protect the body from infections.
CBSE Marking Scheme 2026-27
Mentioning energy for vital biological activities½ Mark
Mentioning body growth and cell division½ Mark
Mentioning repair of worn-out tissues½ Mark
Mentioning protection against infections/diseases½ Mark
Q2
Distinguish between a parasite and a saprotroph.
3 Marks
✅ CBSE Standard Answer
| Feature | Parasite | Saprotroph |
|---|---|---|
| Source of Food | Derives nutrients directly from the living body of another organism called the host. | Absorbs nutrients in solution form from dead and decaying organic matter. |
| Host Impact | Harms the host by depriving it of valuable nutrients and sometimes causes diseases. | Does not harm living organisms; acts as a natural cleaner/decomposer. |
| Digestive Mechanism | Absorbs already digested or synthesised food via specialised sucking roots (haustoria). | Secretes extracellular digestive juices onto dead matter and absorbs the liquified solution. |
| Examples | Cuscuta (Amarbel), tapeworm, mistletoe. | Fungi, mushrooms, yeast, bread mould (Rhizopus). |
CBSE Marking Scheme 2026-27
Difference in source of nutrition (living host vs dead matter)1 Mark
Difference in mode of digestion/host impact1 Mark
Correct examples for both categories1 Mark
Q3
How would you test the presence of starch in leaves?
3 Marks
✅ CBSE Standard Answer
The presence of starch in green leaves is tested using the Iodine Test following a standard laboratory procedure:
Step 1 (Killing the cells): Pluck a green leaf from a healthy plant exposed to sunlight for a few hours. Boil the leaf in a beaker of water for 3 to 5 minutes to break down cell membranes and make it permeable.
Step 2 (Decolourisation): Place the boiled leaf in a test tube containing alcohol (ethanol). Place this test tube inside a water bath (beaker filled with boiling water) and heat gently until all the green pigment (chlorophyll) dissolves out into the alcohol and the leaf turns pale white/colourless.
Caution: Alcohol is highly flammable; never heat it directly over a flame!
Caution: Alcohol is highly flammable; never heat it directly over a flame!
Step 3 (Washing & Softening): Remove the brittle bleached leaf and rinse it gently in warm water to soften it and remove trace alcohol.
Step 4 (Iodine Addition): Spread the decolourised leaf flat on a white ceramic tile and add a few drops of dilute iodine solution onto its surface.
Observation & Conclusion: The leaf turns blue-black in colour. This confirms the presence of starch, proving that photosynthesis has taken place.
CBSE Marking Scheme 2026-27
Boiling in water + alcohol decolourisation in water bath1.5 Marks
Application of iodine solution on bleached leaf0.5 Mark
Blue-black colour observation and valid starch conclusion1 Mark
Q4
Give a brief description of the process of synthesis of food in green plants.
3 Marks
✅ CBSE Standard Answer
Green plants synthesise their food through the physiological process of Photosynthesis. Leaves act as the "food factories" of the plant.
1. Absorption of Water & Minerals: Roots absorb water and dissolved minerals from the soil. These are transported upwards through specialised vascular tissues called xylem vessels to the leaves.
2. Intake of Carbon Dioxide: Carbon dioxide from atmospheric air enters the leaves through microscopic pores called stomata, controlled by guard cells.
3. Trapping Solar Energy: Green leaves contain a special pigment called chlorophyll, which captures radiant solar energy.
4. Chemical Conversion: Solar energy is converted into chemical energy to combine CO₂ and H₂O into carbohydrate (glucose). Oxygen is released as a vital byproduct into the air:
6CO₂ + 6H₂O ⎯⎯⎯[Sunlight / Chlorophyll]⎯⎯→ C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen)
6CO₂ + 6H₂O ⎯⎯⎯[Sunlight / Chlorophyll]⎯⎯→ C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen)
The synthesised glucose is converted and stored in plant organs as starch.
CBSE Marking Scheme 2026-27
Raw materials: water via xylem, CO₂ via stomata1 Mark
Role of chlorophyll and conversion of solar energy1 Mark
Word / Chemical equation with products (glucose + oxygen)1 Mark
Q5
Show with the help of a sketch that plants are the ultimate source of food.
2 Marks
✅ CBSE Standard Answer
All living organisms on Earth depend directly or indirectly on green plants for nourishment, as plants alone convert solar energy into chemical energy:
☀️ Sun (Ultimate Source of Energy)
↓ (trapped by chlorophyll)
🌱 Green Plants / Producers (Autotrophs)
↓ (consumed by herbivores)
🦗 Herbivores / Primary Consumers (e.g., Deer, Grasshopper, Goat)
↓ (consumed by carnivores)
🦁 Carnivores / Secondary Consumers (e.g., Lion, Frog, Eagle)
↓ (on death of organisms)
🍄 Decomposers / Saprotrophs (Bacteria, Fungi) → Soil Nutrients → Plants
↓ (trapped by chlorophyll)
🌱 Green Plants / Producers (Autotrophs)
↓ (consumed by herbivores)
🦗 Herbivores / Primary Consumers (e.g., Deer, Grasshopper, Goat)
↓ (consumed by carnivores)
🦁 Carnivores / Secondary Consumers (e.g., Lion, Frog, Eagle)
↓ (on death of organisms)
🍄 Decomposers / Saprotrophs (Bacteria, Fungi) → Soil Nutrients → Plants
Explanation: Herbivores directly eat plants. Carnivores eat herbivores that fed on plants. If plants did not exist, herbivores would starve, leading to the extinction of carnivores. Thus, plants are the ultimate source of all food on Earth.
CBSE Marking Scheme 2026-27
Clear sequential sketch (Sun → Plants → Herbivores → Carnivores)1 Mark
Scientific explanation of direct/indirect dependence1 Mark
Q6
Fill in the blanks:
(a) Green plants are called _________ since they synthesise their own food.
(b) The food synthesised by plants is stored as _________.
(c) In photosynthesis solar energy is absorbed by the pigment called _________.
(d) During photosynthesis plants take in _________ and release _________ gas.
(a) Green plants are called _________ since they synthesise their own food.
(b) The food synthesised by plants is stored as _________.
(c) In photosynthesis solar energy is absorbed by the pigment called _________.
(d) During photosynthesis plants take in _________ and release _________ gas.
2 Marks
✅ CBSE Standard Answer
(a) Green plants are called autotrophs since they synthesise their own food.
(b) The food synthesised by plants is stored as starch.
(c) In photosynthesis solar energy is absorbed by the pigment called chlorophyll.
(d) During photosynthesis plants take in carbon dioxide and release oxygen gas.
CBSE Marking Scheme 2026-27
Each correct blank (0.5 Mark × 4)2 Marks
Q7
Name the following:
(i) A parasitic plant with yellow, slender and branched stem.
(ii) A plant that is partially autotrophic.
(iii) The pores through which leaves exchange gases.
(i) A parasitic plant with yellow, slender and branched stem.
(ii) A plant that is partially autotrophic.
(iii) The pores through which leaves exchange gases.
3 Marks
✅ CBSE Standard Answer
(i) Cuscuta (Amarbel) — A total parasite that lacks chlorophyll and wraps around host trees.
(ii) Pitcher plant (Nepenthes) or Venus flytrap — Possesses green chlorophyll to perform photosynthesis, but traps insects to supplement nitrogen.
(iii) Stomata — Microscopic apertures on leaf surfaces surrounded by guard cells.
CBSE Marking Scheme 2026-27
Each correct name (1 Mark × 3)3 Marks
Q8
Tick the correct answer:
(a) Cuscuta is an example of: (i) autotroph (ii) parasite (iii) saprotroph (iv) host
(b) The plant which traps and feeds on insects is: (i) Cuscuta (ii) china rose (iii) pitcher plant (iv) rose
(a) Cuscuta is an example of: (i) autotroph (ii) parasite (iii) saprotroph (iv) host
(b) The plant which traps and feeds on insects is: (i) Cuscuta (ii) china rose (iii) pitcher plant (iv) rose
2 Marks
✅ CBSE Standard Answer
(a) (ii) parasite
Reason: Cuscuta climbs on other trees and derives its food from the host plant using haustoria.
Reason: Cuscuta climbs on other trees and derives its food from the host plant using haustoria.
(b) (iii) pitcher plant
Reason: Its leaf blade is modified into a jug-like pitcher with a lid to trap insects and digest them with enzymes.
Reason: Its leaf blade is modified into a jug-like pitcher with a lid to trap insects and digest them with enzymes.
CBSE Marking Scheme 2026-27
Part (a): (ii) parasite1 Mark
Part (b): (iii) pitcher plant1 Mark
Q9
Match the items given in Column I with those in Column II:
Column I: Chlorophyll, Nitrogen, Cuscuta, Animals, Insects
Column II: Rhizobium, Heterotrophs, Pitcher plant, Leaf, Parasite
Column I: Chlorophyll, Nitrogen, Cuscuta, Animals, Insects
Column II: Rhizobium, Heterotrophs, Pitcher plant, Leaf, Parasite
2.5 Marks
✅ CBSE Standard Answer
| Column I | Column II | Scientific Rationale |
|---|---|---|
| Chlorophyll | Leaf | Found in chloroplasts of leaf mesophyll cells. |
| Nitrogen | Rhizobium | Rhizobium fixes atmospheric nitrogen into soluble nitrates in root nodules. |
| Cuscuta | Parasite | Non-green climber that absorbs readymade food from host trees. |
| Animals | Heterotrophs | Cannot make own food; depend on plants and other animals. |
| Insects | Pitcher plant | Pitcher plant traps and digests insects for nitrogen requirements. |
CBSE Marking Scheme 2026-27
Each correct pair matched accurately (0.5 Mark × 5)2.5 Marks
Q10
Mark 'T' if the statement is true and 'F' if it is false:
(i) Carbon dioxide is released during photosynthesis. (T/F)
(ii) Plants which synthesise their food are called saprotrophs. (T/F)
(iii) The product of photosynthesis is not a protein. (T/F)
(iv) Solar energy is converted into chemical energy during photosynthesis. (T/F)
(i) Carbon dioxide is released during photosynthesis. (T/F)
(ii) Plants which synthesise their food are called saprotrophs. (T/F)
(iii) The product of photosynthesis is not a protein. (T/F)
(iv) Solar energy is converted into chemical energy during photosynthesis. (T/F)
2 Marks
✅ CBSE Standard Answer
(i) False (F) — Oxygen is released during photosynthesis; carbon dioxide is taken in.
(ii) False (F) — Plants that synthesise their own food are called autotrophs (saprotrophs feed on dead decaying matter).
(iii) True (T) — The primary product of photosynthesis is a carbohydrate (glucose/starch), not a protein.
(iv) True (T) — Solar light energy is converted into chemical bond energy stored in glucose molecules.
CBSE Marking Scheme 2026-27
Each correct True/False determination (0.5 Mark × 4)2 Marks
Q11
Choose the correct option: Which part of the plant takes in carbon dioxide from the air for photosynthesis? (i) Root hair (ii) Stomata (iii) Leaf veins (iv) Petals
1 Mark
✅ CBSE Standard Answer
Correct Option: (ii) Stomata
Scientific Explanation: Stomata are microscopic pores located primarily on the lower epidermal surface of leaves. They are regulated by guard cells that open during daylight to allow the diffusion of atmospheric CO₂ into photosynthetic mesophyll tissues.
CBSE Marking Scheme 2026-27
Option (ii) Stomata with brief explanation1 Mark
Q12
Choose the correct option: Plants take carbon dioxide from the atmosphere mainly through their: (i) roots (ii) stem (iii) flowers (iv) leaves
1 Mark
✅ CBSE Standard Answer
Correct Option: (iv) leaves
Scientific Explanation: Leaves bear the vast majority of stomata and contain the green mesophyll tissues where photosynthesis occurs, making them the primary site of atmospheric carbon dioxide intake.
CBSE Marking Scheme 2026-27
Option (iv) leaves1 Mark
Q13
Why do farmers grow many fruits and vegetable crops inside large green houses? What are the advantages to the farmers?
3 Marks
✅ CBSE Standard Answer
A greenhouse (glasshouse or polyhouse) provides a controlled microenvironment for plant cultivation. Farmers grow crops inside greenhouses due to the following specific advantages:
1. Optimal Temperature & Light Control: Glass walls trap solar heat (greenhouse effect), maintaining a warmer, optimum temperature for photosynthesis even during cold winter seasons.
2. Protection from Harsh Weather & Pests: Enclosed structures shield delicate crops from high winds, hailstorms, heavy rainfall, frost, birds, insects, and air-borne pests.
3. Controlled Carbon Dioxide & Humidity: Farmers can enrich the internal air with higher concentrations of CO₂ and maintain ideal humidity, leading to faster growth rates, higher yields, and better-quality produce.
4. Year-Round Farming: It enables farmers to grow off-season vegetables and fruits, fetching premium market prices.
CBSE Marking Scheme 2026-27
Greenhouse effect trapping warmth for photosynthesis1 Mark
Protection from pests, wind, frost and weather extremes1 Mark
Controlled CO₂/humidity and off-season yield advantages1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Nitrogen Fixation in Agriculture
Scenario: Farmer Ramesh noticed that after harvesting wheat, the soil becomes depleted of nitrogen. However, his agricultural advisor recommended rotating the crop by planting gram (chickpea) or peas before sowing wheat again, without applying synthetic nitrogen fertilisers.
(i) Name the bacterium that lives in the roots of gram and pea plants.
(ii) What type of relationship exists between this bacterium and the leguminous plant?
(iii) Why is crop rotation with legumes economically and ecologically beneficial?
(i) Name the bacterium that lives in the roots of gram and pea plants.
(ii) What type of relationship exists between this bacterium and the leguminous plant?
(iii) Why is crop rotation with legumes economically and ecologically beneficial?
(i) Bacterium Name: Rhizobium.
(ii) Biological Relationship: Symbiotic relationship (Symbiosis). The bacterium cannot synthesise its own food; it receives shelter, carbohydrates, and organic nutrients from the host plant roots. In exchange, Rhizobium converts atmospheric gaseous nitrogen (N₂) into water-soluble nitrogenous compounds (nitrates) that the plant easily absorbs.
(iii) Benefits: (a) Economic: Saves farmers significant money on expensive chemical fertilisers. (b) Ecological: Prevents soil degradation, preserves soil texture, and stops chemical runoff from polluting nearby water bodies (eutrophication).
⚡ Assertion & Reason (HOTS)
Assertion (A): Pitcher plant performs photosynthesis but still traps and digests insects.
Reason (R): Pitcher plants grow in nitrogen-deficient boggy soils and feed on insects to fulfil their nitrogen requirement.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Pitcher plants grow in nitrogen-deficient boggy soils and feed on insects to fulfil their nitrogen requirement.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Pitcher plants possess green chlorophyll and synthesise their own carbohydrates through photosynthesis. However, the waterlogged, marshy soils they inhabit are severely deficient in available nitrogen minerals. To synthesise essential proteins, the leaf lamina modifies into a pitcher trap with digestive enzymes to digest trapped insects and absorb amino acids.
2
Chapter 2: Nutrition in Animals
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Digestive System Summary
- Five Major Steps of Holozoic Nutrition:
- Ingestion: Taking food into the body via the mouth (buccal cavity).
- Digestion: Breakdown of complex, insoluble food molecules into simple, soluble forms by digestive enzymes.
- Absorption: Movement of digested nutrients through intestinal walls (villi) into the bloodstream.
- Assimilation: Utilization of absorbed nutrients by body cells for energy, growth, and tissue repair.
- Egestion: Removal of undigested, unabsorbed waste residue from the body through the anus.
- Human Digestive Canal & Associated Glands:
- Mouth & Teeth: Incisors (cutting/biting), Canines (tearing), Premolars & Molars (chewing/grinding). Salivary amylase breaks starch into maltose.
- Stomach: Secretes mucous (protects inner wall), hydrochloric acid (kills bacteria, creates acidic medium pH ~1.5–2), and pepsin (digests proteins into peptides).
- Liver: Largest gland in human body; secretes bile stored in gall bladder; emulsifies fats (breaks large fat globules into tiny droplets).
- Pancreas: Secretes pancreatic juice containing amylase, trypsin, and lipase.
- Small Intestine: Longest part (~7.5 m); site of complete digestion and nutrient absorption through finger-like villi.
- Large Intestine: ~1.5 m; absorbs water and essential salts from undigested residue.
- Nutrition in Ruminants: Four-chambered stomach (Rumen, Reticulum, Omasum, Abomasum). Swallowed food is stored in rumen as cud, brought back to mouth and chewed (rumination). Symbiotic bacteria in caecum digest complex cellulose.
- Nutrition in Amoeba: Unicellular organism that pushes out finger-like projections called pseudopodia (false feet) to engulf food into a food vacuole where digestive enzymes break it down.
NCERT Intext Questions & Scientific Enquiries
Page 11 — Intext Q1
What are the different modes of taking food into the body in different animals? Give examples.
2 Marks
✅ CBSE Standard Answer
Different animal species have evolved specialized feeding mechanisms depending on their body structure and diet:
- Chewing & Biting: Humans, dogs, cattle break down solid food with specialized teeth.
- Scraping: Snails use a toothed ribbon (radula) to scrape algae from rocks.
- Siphoning: Butterflies and moths suck nectar from flowers using a coiled tubular proboscis.
- Sponging: Houseflies liquefy food with saliva and sponge it up with a fleshy labellum.
- Swallowing whole: Snakes (e.g. pythons) swallow prey whole without chewing.
- Filter Feeding: Aquatic animals like flamingos and baleen whales filter tiny food particles floating in water.
CBSE Marking Scheme 2026-27
Any 4 distinct feeding modes identified with accurate examples2 Marks
Page 15 — Intext Q2 (Paheli's Query)
Paheli wants to know how food moves in the opposite direction during vomiting.
2 Marks
✅ CBSE Standard Answer
Mechanism of Vomiting (Anti-peristalsis):
1. Normally, food moves downward through the alimentary canal by rhythmic wave-like contractions of the muscular gut wall called peristalsis.
2. When the stomach detects toxic, contaminated, or irritating food, or when it is excessively overloaded, nerve signals trigger the vomiting reflex in the brainstem.
3. The normal downward peristalsis reverses into anti-peristalsis (retro-peristalsis).
4. The stomach and abdominal muscles violently contract simultaneously, the cardiac sphincter at the esophageal entrance relaxes, and the stomach contents are forcefully propelled backwards up the esophagus and expelled through the mouth.
1. Normally, food moves downward through the alimentary canal by rhythmic wave-like contractions of the muscular gut wall called peristalsis.
2. When the stomach detects toxic, contaminated, or irritating food, or when it is excessively overloaded, nerve signals trigger the vomiting reflex in the brainstem.
3. The normal downward peristalsis reverses into anti-peristalsis (retro-peristalsis).
4. The stomach and abdominal muscles violently contract simultaneously, the cardiac sphincter at the esophageal entrance relaxes, and the stomach contents are forcefully propelled backwards up the esophagus and expelled through the mouth.
CBSE Marking Scheme 2026-27
Normal downward peristalsis vs reverse anti-peristalsis1 Mark
Forceful contraction of stomach and abdominal muscles expelling food1 Mark
Page 17 — Intext Q3 (Boojho's Query)
Boojho wants to know why ruminants (like cows and buffaloes) cannot chew their food properly when they take it in, but chew cud later while resting.
2 Marks
✅ CBSE Standard Answer
Evolutionary Adaptation of Ruminants:
1. Predator Avoidance: In the wild, grazing herbivores are vulnerable to carnivores in open pastures. To minimize time exposed, they swallow huge quantities of grass as quickly as possible with minimal chewing.
2. Storage in Rumen: The swallowed grass is stored in the rumen (first stomach chamber), where symbiotic anaerobic bacteria and protozoa begin breaking down tough cellulose, forming soft partially digested food called cud.
3. Rumination in Safety: Once the animal retreats to a sheltered, safe spot to rest, small lumps of cud are regurgitated back into the mouth in batches and thoroughly chewed (chewing the cud / rumination). This fine mastication enables complete microbial fermentation later in the digestive tract.
1. Predator Avoidance: In the wild, grazing herbivores are vulnerable to carnivores in open pastures. To minimize time exposed, they swallow huge quantities of grass as quickly as possible with minimal chewing.
2. Storage in Rumen: The swallowed grass is stored in the rumen (first stomach chamber), where symbiotic anaerobic bacteria and protozoa begin breaking down tough cellulose, forming soft partially digested food called cud.
3. Rumination in Safety: Once the animal retreats to a sheltered, safe spot to rest, small lumps of cud are regurgitated back into the mouth in batches and thoroughly chewed (chewing the cud / rumination). This fine mastication enables complete microbial fermentation later in the digestive tract.
CBSE Marking Scheme 2026-27
Rapid ingestion to avoid predators in open grazing fields1 Mark
Storage in rumen to form cud followed by detailed chewing in safety1 Mark
Page 18 — Intext Q4 (Boojho's Query)
Boojho wants to know why humans cannot digest cellulose like cattle and horses do.
2 Marks
✅ CBSE Standard Answer
Why Humans Cannot Digest Cellulose:
1. Lack of Cellulase Enzyme: Cellulose is a tough, complex structural carbohydrate with strong β-glycosidic linkages. Humans do not produce the enzyme cellulase required to cleave these bonds.
2. Absence of Cellulolytic Symbionts: Herbivorous ruminants have a rumen and animals like horses and rabbits have an enlarged caecum hosting millions of specialized symbiotic anaerobic bacteria and protozoa that ferment and digest cellulose.
3. Vestigial Human Caecum: In humans, the caecum is reduced, and the appendix is a tiny vestigial organ with no cellulose-digesting bacteria. Hence, cellulose passes through the human gut undigested as dietary fiber (roughage).
1. Lack of Cellulase Enzyme: Cellulose is a tough, complex structural carbohydrate with strong β-glycosidic linkages. Humans do not produce the enzyme cellulase required to cleave these bonds.
2. Absence of Cellulolytic Symbionts: Herbivorous ruminants have a rumen and animals like horses and rabbits have an enlarged caecum hosting millions of specialized symbiotic anaerobic bacteria and protozoa that ferment and digest cellulose.
3. Vestigial Human Caecum: In humans, the caecum is reduced, and the appendix is a tiny vestigial organ with no cellulose-digesting bacteria. Hence, cellulose passes through the human gut undigested as dietary fiber (roughage).
CBSE Marking Scheme 2026-27
Absence of cellulase enzyme in human digestive secretions1 Mark
Absence of symbiotic cellulose-fermenting rumen/caecal microbes in humans1 Mark
NCERT Textbook Exercise Questions
Q1
Fill in the blanks:
(a) The main steps of nutrition in humans are _________, _________, _________, _________ and _________.
(b) The largest gland in the human body is _________.
(c) The stomach releases hydrochloric acid and _________ juices which act on food.
(d) The inner wall of the small intestine has many finger-like outgrowths called _________.
(e) Amoeba digests its food in the _________.
(a) The main steps of nutrition in humans are _________, _________, _________, _________ and _________.
(b) The largest gland in the human body is _________.
(c) The stomach releases hydrochloric acid and _________ juices which act on food.
(d) The inner wall of the small intestine has many finger-like outgrowths called _________.
(e) Amoeba digests its food in the _________.
2.5 Marks
✅ CBSE Standard Answer
(a) The main steps of nutrition in humans are ingestion, digestion, absorption, assimilation and egestion.
(b) The largest gland in the human body is liver.
(c) The stomach releases hydrochloric acid and digestive juices which act on food.
(d) The inner wall of the small intestine has many finger-like outgrowths called villi.
(e) Amoeba digests its food in the food vacuole.
CBSE Marking Scheme 2026-27
Each correct blank (0.5 Mark × 5)2.5 Marks
Q2
Mark 'T' if the statement is true and 'F' if it is false:
(a) Digestion of starch starts in the stomach. (T/F)
(b) The tongue helps in mixing food with saliva. (T/F)
(c) The gall bladder temporarily stores bile. (T/F)
(d) The ruminants bring back swallowed grass into their mouth and chew it for some time. (T/F)
(a) Digestion of starch starts in the stomach. (T/F)
(b) The tongue helps in mixing food with saliva. (T/F)
(c) The gall bladder temporarily stores bile. (T/F)
(d) The ruminants bring back swallowed grass into their mouth and chew it for some time. (T/F)
2 Marks
✅ CBSE Standard Answer
(a) False (F) — Digestion of starch starts in the mouth (buccal cavity) by the action of salivary amylase.
(b) True (T) — The muscular tongue moves food around, mixes it thoroughly with saliva, and assists in swallowing.
(c) True (T) — Bile juice synthesised by the liver is stored and concentrated in the gall bladder until needed.
(d) True (T) — Ruminants quickly swallow vegetation into the rumen, then bring the partially digested cud back into the mouth for thorough rumination.
CBSE Marking Scheme 2026-27
Each correct evaluation with brief scientific rationale0.5 Mark each (Total 2 Marks)
Q3
Tick the correct answer:
(a) Fat is completely digested in the: (i) stomach (ii) mouth (iii) small intestine (iv) large intestine
(b) Water from the undigested food is absorbed mainly in the: (i) stomach (ii) foodpipe (iii) small intestine (iv) large intestine
(a) Fat is completely digested in the: (i) stomach (ii) mouth (iii) small intestine (iv) large intestine
(b) Water from the undigested food is absorbed mainly in the: (i) stomach (ii) foodpipe (iii) small intestine (iv) large intestine
2 Marks
✅ CBSE Standard Answer
(a) (iii) small intestine
Reason: In the small intestine, bile from liver emulsifies fats, and pancreatic lipase together with intestinal lipases breaks them down completely into fatty acids and glycerol.
Reason: In the small intestine, bile from liver emulsifies fats, and pancreatic lipase together with intestinal lipases breaks them down completely into fatty acids and glycerol.
(b) (iv) large intestine
Reason: The large intestine absorbs water and remaining minerals from undigested food slurry, forming semi-solid faeces stored in the rectum.
Reason: The large intestine absorbs water and remaining minerals from undigested food slurry, forming semi-solid faeces stored in the rectum.
CBSE Marking Scheme 2026-27
Part (a): (iii) small intestine1 Mark
Part (b): (iv) large intestine1 Mark
Q4
Match the items of Column I with those given in Column II:
Column I (Food components): Carbohydrates, Proteins, Fats
Column II (Products of digestion): Fatty acids and glycerol, Sugar (glucose), Amino acids
Column I (Food components): Carbohydrates, Proteins, Fats
Column II (Products of digestion): Fatty acids and glycerol, Sugar (glucose), Amino acids
3 Marks
✅ CBSE Standard Answer
| Column I (Complex Food Component) | Column II (End Product of Digestion) |
|---|---|
| Carbohydrates | Sugar (Glucose) |
| Proteins | Amino acids |
| Fats | Fatty acids and glycerol |
CBSE Marking Scheme 2026-27
Each correct matching pair (1 Mark × 3)3 Marks
Q5
What are villi? What is their location and function?
3 Marks
✅ CBSE Standard Answer
Definition: Villi (singular: villus) are millions of microscopic, finger-like projections protruding from the mucous membrane of the digestive tract.
Location: Villi are located on the inner mucosal lining of the small intestine (especially in the jejunum and ileum).
Functions:
- Increasing Surface Area: The millions of villi vastly increase the effective absorptive surface area of the small intestine by hundreds of times.
- Rapid Nutrient Absorption: Each villus contains a dense network of thin, fine blood capillaries and a central lymph vessel (lacteal). Simple digested nutrients (glucose, amino acids, minerals, vitamins) readily diffuse through the thin single-cell epithelium into the blood stream for transportation to all body cells.
CBSE Marking Scheme 2026-27
Definition of villi (finger-like outgrowths)1 Mark
Location: Inner wall of small intestine1 Mark
Function: Maximising surface area for rapid nutrient absorption1 Mark
Q6
Where is bile produced? Which component of the food does it help to digest?
2 Marks
✅ CBSE Standard Answer
Site of Production & Storage: Bile juice is produced continuously by the Liver. It is temporarily stored and concentrated in a sac-like organ called the Gall Bladder.
Component of Food Digested: Bile plays a vital role in the digestion of Fats (Lipids).
Mechanism (Emulsification): Bile does not contain enzymes, but its bile salts break down large, hydrophobic fat globules into microscopic droplets (a process called emulsification). This enormously increases the surface area for pancreatic lipase enzymes to chemically break fats into fatty acids and glycerol. Furthermore, bile provides the necessary alkaline medium for pancreatic enzymes to function.
CBSE Marking Scheme 2026-27
Production site: Liver (stored in gall bladder)1 Mark
Digests fats via emulsification1 Mark
Q7
Name the type of carbohydrate that can be digested by ruminants but not by humans. Give the reason also.
3 Marks
✅ CBSE Standard Answer
Carbohydrate Name: Cellulose (the tough structural polysaccharide constituting plant cell walls and grass).
Scientific Reason:
- In Ruminants (Cattle, Deer, Buffaloes): Ruminants possess a specialised large sac-like chamber called the caecum / rumen between the small and large intestine. Inside this chamber live specialised symbiotic anaerobic bacteria and protozoa that secrete the enzyme cellulase, which hydrolyses the tough β-glycosidic bonds of cellulose.
- In Humans: The human digestive system completely lacks the enzyme cellulase as well as the symbiotic bacteria required to break down cellulose. Therefore, cellulose passes through the human alimentary canal undigested, functioning only as dietary fibre (roughage) to aid bowel movement.
CBSE Marking Scheme 2026-27
Identifying carbohydrate as Cellulose1 Mark
Presence of cellulose-digesting bacteria in ruminants1 Mark
Absence of cellulase enzyme/bacteria in humans1 Mark
Q8
Why do we get instant energy from glucose?
2 Marks
✅ CBSE Standard Answer
1. No Digestion Required: Glucose is a simple monosaccharide (the simplest form of carbohydrate). Unlike complex carbohydrates (starch, sucrose) which require hours of enzymatic digestion in the mouth, stomach, and intestine to be broken down, glucose requires zero digestion.
2. Direct & Immediate Absorption: When consumed, glucose is instantly absorbed directly across the intestinal wall into the bloodstream and delivered straight to body cells.
3. Rapid Cellular Respiration: Inside cells, glucose is readily oxidised by oxygen during cellular respiration to immediately release energy in the form of ATP:
Glucose + Oxygen ⎯⎯→ Carbon dioxide + Water + Energy (ATP)
Glucose + Oxygen ⎯⎯→ Carbon dioxide + Water + Energy (ATP)
CBSE Marking Scheme 2026-27
Glucose is simplest sugar requiring no prior digestion1 Mark
Direct absorption into blood and instant oxidation in cells1 Mark
Q9
Which part of the digestive canal is involved in:
(i) absorption of food _________.
(ii) chewing of food _________.
(iii) killing of bacteria _________.
(iv) complete digestion of food _________.
(v) formation of faeces _________.
(i) absorption of food _________.
(ii) chewing of food _________.
(iii) killing of bacteria _________.
(iv) complete digestion of food _________.
(v) formation of faeces _________.
2.5 Marks
✅ CBSE Standard Answer
(i) Absorption of food: Small intestine (via intestinal villi).
(ii) Chewing of food: Buccal cavity (Mouth) (by teeth).
(iii) Killing of bacteria: Stomach (by hydrochloric acid).
(iv) Complete digestion of food: Small intestine (action of bile, pancreatic, and intestinal juices).
(v) Formation of faeces: Large intestine (water absorbed, compacted into solid faeces).
CBSE Marking Scheme 2026-27
Each correct organ identified (0.5 Mark × 5)2.5 Marks
Q10
State one similarity and one difference between the nutrition in amoeba and human beings.
2 Marks
✅ CBSE Standard Answer
Similarity: Both Amoeba and humans exhibit holozoic nutrition consisting of the same fundamental sequence of stages: ingestion, digestion (where complex substances are broken down into simpler soluble forms by digestive enzymes), absorption, assimilation for energy, and egestion of undigested waste.
Difference:
- Amoeba: Being a single-celled microscopic organism, it has no mouth or specialised digestive canal; food is captured by pseudopodia and digested inside a temporary food vacuole.
- Humans: Being complex multicellular organisms, they possess a specialised, highly developed alimentary canal with distinct organs (mouth, stomach, intestine) and specialised digestive glands (salivary glands, liver, pancreas).
CBSE Marking Scheme 2026-27
Valid similarity (holozoic stages / enzymatic digestion)1 Mark
Valid structural difference (food vacuole vs complex alimentary canal)1 Mark
Q11
Match the items of Column I with suitable items in Column II:
Column I: (a) Salivary gland, (b) Stomach, (c) Liver, (d) Rectum, (e) Small intestine, (f) Large intestine
Column II: (i) Bile juice secretion, (ii) Storage of undigested food, (iii) Saliva secretion, (iv) Acid release, (v) Digestion is completed, (vi) Absorption of water
Column I: (a) Salivary gland, (b) Stomach, (c) Liver, (d) Rectum, (e) Small intestine, (f) Large intestine
Column II: (i) Bile juice secretion, (ii) Storage of undigested food, (iii) Saliva secretion, (iv) Acid release, (v) Digestion is completed, (vi) Absorption of water
3 Marks
✅ CBSE Standard Answer
| Column I (Organ / Gland) | Column II (Primary Function) |
|---|---|
| (a) Salivary gland | (iii) Saliva secretion |
| (b) Stomach | (iv) Acid release (HCl) |
| (c) Liver | (i) Bile juice secretion |
| (d) Rectum | (ii) Storage of undigested food |
| (e) Small intestine | (v) Digestion is completed |
| (f) Large intestine | (vi) Absorption of water |
CBSE Marking Scheme 2026-27
Each correct pair accurately matched (0.5 Mark × 6)3 Marks
Q12
Label the following parts of the human digestive system in sequence: Stomach, Gall bladder, Liver, Pancreas, Small intestine, Large intestine, Rectum, Anus.
3 Marks
✅ CBSE Standard Answer
The anatomical sequence of organs forming the human digestive system from mouth to anus is:
1. Mouth / Buccal Cavity: Teeth masticate food; salivary glands add saliva.
2. Oesophagus (Foodpipe): Connects pharynx to stomach through peristaltic waves.
3. Stomach: J-shaped muscular sac on the upper left abdomen that releases HCl, mucous, and pepsin.
4. Liver: Large reddish-brown gland on upper right side; produces bile.
5. Gall Bladder: Pear-shaped sac nestled beneath liver; stores and concentrates bile.
6. Pancreas: Cream-coloured leaf-shaped gland located just below stomach; secretes pancreatic enzymes.
7. Small Intestine: Highly coiled 7.5-metre central organ; completes chemical digestion and absorbs nutrients via villi.
8. Large Intestine: Wider 1.5-metre frame surrounding the small intestine; absorbs water and salts.
9. Rectum: Terminal pouch storing compacted faeces.
10. Anus: Muscular sphincter opening for egestion.
2. Oesophagus (Foodpipe): Connects pharynx to stomach through peristaltic waves.
3. Stomach: J-shaped muscular sac on the upper left abdomen that releases HCl, mucous, and pepsin.
4. Liver: Large reddish-brown gland on upper right side; produces bile.
5. Gall Bladder: Pear-shaped sac nestled beneath liver; stores and concentrates bile.
6. Pancreas: Cream-coloured leaf-shaped gland located just below stomach; secretes pancreatic enzymes.
7. Small Intestine: Highly coiled 7.5-metre central organ; completes chemical digestion and absorbs nutrients via villi.
8. Large Intestine: Wider 1.5-metre frame surrounding the small intestine; absorbs water and salts.
9. Rectum: Terminal pouch storing compacted faeces.
10. Anus: Muscular sphincter opening for egestion.
CBSE Marking Scheme 2026-27
Correct identification and anatomical sequence of organs3 Marks
Q13
Can we survive only on raw, leafy vegetables/grass? Discuss.
3 Marks
✅ CBSE Standard Answer
No, human beings cannot survive solely on raw, leafy vegetables and grass.
1. Inability to Digest Cellulose: Grass and raw leafy plants primarily contain cellulose, a tough structural carbohydrate. The human digestive tract completely lacks the enzyme cellulase and does not house the symbiotic cellulose-fermenting micro-organisms found in ruminants (cows, goats). Consequently, cellulose passes out undigested, providing virtually zero energy to humans.
2. Severe Energy & Nutrient Deficit: Raw vegetables contain very high proportions of water and fibre, but extremely low concentrations of accessible proteins, essential fats, and complex carbohydrates needed to sustain human metabolic rates and body organ functions.
3. Nutritional Role as Roughage: While leafy vegetables are vital in a balanced diet to provide essential vitamins, minerals, and roughage (fibre) to maintain healthy bowel movement and prevent constipation, they must be supplemented with energy-dense foods (grains, pulses, dairy, oils) to sustain life.
CBSE Marking Scheme 2026-27
Clear negative response stating lack of cellulase in humans1 Mark
Inadequate calorie, protein, and fat provision from grass1 Mark
Highlighting role of leafy greens as roughage/vitamins in a balanced diet1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Oral Rehydration Therapy (ORT)
Clinical Case: 10-year-old Aarav contracted an intestinal infection after eating unhygienic street food. He experienced frequent watery stools and vomiting, feeling extremely weak, dizzy, and parched. The doctor diagnosed him with acute diarrhoea, warned against severe dehydration, and immediately prescribed frequent sips of ORS (Oral Rehydration Solution).
(i) What is dehydration, and why is it dangerous?
(ii) How can an emergency ORS solution be prepared at home?
(iii) Why is simple plain water not as effective as ORS during severe diarrhoea?
(i) What is dehydration, and why is it dangerous?
(ii) How can an emergency ORS solution be prepared at home?
(iii) Why is simple plain water not as effective as ORS during severe diarrhoea?
(i) Dehydration: Dehydration is the excessive loss of water and vital dissolved mineral salts (electrolytes such as sodium, potassium, and chloride) from body tissues. It is life-threatening because blood volume drops, leading to hypovolemic shock, organ failure, and collapse.
(ii) Home Preparation of ORS: Boil 1 litre of clean drinking water and let it cool. Dissolve 6 level teaspoons of sugar and half a level teaspoon of salt thoroughly in the water.
(iii) Mechanism: The intestinal lining absorbs sodium and water through co-transport coupled with glucose molecules. Plain water cannot be efficiently absorbed by an inflamed, rapidly purging bowel and gets excreted. The presence of glucose in ORS triggers rapid intestinal sodium and water reabsorption back into the circulatory system.
⚡ Assertion & Reason (HOTS)
Assertion (A): A patient whose gall bladder has been surgically removed is advised to avoid heavy, fatty meals.
Reason (R): The gall bladder produces bile juice, and without it, no bile is synthesised in the body.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): The gall bladder produces bile juice, and without it, no bile is synthesised in the body.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (c) A is true, but R is false.
Scientific Explanation: Assertion (A) is true because without the gall bladder, there is no reservoir to store and release concentrated pulses of bile when a fatty meal enters the duodenum. However, Reason (R) is completely false: bile is synthesised by the liver, not the gall bladder. Following a cholecystectomy, the liver continues to produce bile, which drips continuously in dilute amounts directly into the intestine, making large boluses of fat difficult to digest.
3
Chapter 3: Heat
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Thermodynamics Summary
- Heat vs Temperature:
- Heat: A form of energy that flows spontaneously from a body at a higher temperature to a body at a lower temperature until thermal equilibrium is reached. Unit: Joule (J) or Calorie (cal).
- Temperature: A reliable quantitative measure of the degree of hotness or coldness of an object. Measured using thermometers in Degree Celsius (°C), Fahrenheit (°F), or Kelvin (K).
- Thermometers:
- Clinical Thermometer: Designed specifically for human body temperature. Range: 35°C to 42°C (or 94°F to 108°F). Contains a narrow constriction (kink) just above the bulb to prevent mercury from falling back automatically before the reading is recorded.
- Laboratory Thermometer: Used to measure temperature of chemical solutions and substances in labs. Range: -10°C to 110°C. Has no kink; reading must be taken while still in contact with the substance.
- Three Modes of Heat Transfer:
- Conduction: Heat transfer in solids from the hotter end to the colder end through molecular vibrations without the actual movement of particles. Metals are good conductors; wood, plastic, glass, and air are insulators.
- Convection: Heat transfer in fluids (liquids and gases) where warmer, less dense fluid expands, rises, and cooler denser fluid sinks to replace it, setting up convection currents. Drives sea breeze (day) and land breeze (night) in coastal areas.
- Radiation: Heat transfer through electromagnetic infrared waves without requiring any physical medium. Can travel through vacuum (how the Sun's heat warms Earth). Dark, dull surfaces absorb and radiate heat best; light, polished surfaces reflect heat.
NCERT Intext Questions & Scientific Enquiries
Page 35 — Intext Q1 (Paheli's Query)
Paheli tested three mugs of water: Left hand in cold water (Mug A), Right hand in hot water (Mug B), then both hands in lukewarm water (Mug C). Her left hand feels Mug C is hot, while right hand feels it is cold! What should she conclude? Can we rely on our sense of touch to measure temperature?
2 Marks
✅ CBSE Standard Answer
Scientific Conclusion:
1. Sense of Touch is Relative & Deceptive: Our skin's thermal receptors do not measure absolute temperature; they only detect the direction and rate of heat transfer relative to the hand's previous temperature.
2. For the cold left hand, heat flows from lukewarm water into the hand, so it feels 'hot'. For the hot right hand, heat flows out of the hand into the water, so it feels 'cold'.
3. Conclusion: We cannot rely on our sense of touch to determine the temperature of an object accurately. A reliable physical instrument — a thermometer — is essential to measure true thermal energy.
1. Sense of Touch is Relative & Deceptive: Our skin's thermal receptors do not measure absolute temperature; they only detect the direction and rate of heat transfer relative to the hand's previous temperature.
2. For the cold left hand, heat flows from lukewarm water into the hand, so it feels 'hot'. For the hot right hand, heat flows out of the hand into the water, so it feels 'cold'.
3. Conclusion: We cannot rely on our sense of touch to determine the temperature of an object accurately. A reliable physical instrument — a thermometer — is essential to measure true thermal energy.
CBSE Marking Scheme 2026-27
Explanation of relative heat gain vs heat loss deceiving the skin1 Mark
Conclusion that sense of touch is unreliable; thermometer required1 Mark
Page 37 — Intext Q2 (Paheli's Query)
Paheli asks: Can we use a clinical thermometer to measure the temperature of boiling milk or hot water? Why or why not?
2 Marks
✅ CBSE Standard Answer
Answer: No, a clinical thermometer should NEVER be used to measure boiling milk or hot liquids.
Reasons:
1. Limited Temperature Range: A clinical thermometer is calibrated strictly for the human body temperature range from 35°C to 42°C (or 94°F to 108°F).
2. Risk of Glass Bursting: Boiling milk or boiling water reaches 100°C or higher. When exposed to this temperature, mercury expands violently beyond the top of the capillary bore, causing the glass bulb and stem to shatter explosively, creating severe burn hazards and toxic mercury vapor exposure.
1. Limited Temperature Range: A clinical thermometer is calibrated strictly for the human body temperature range from 35°C to 42°C (or 94°F to 108°F).
2. Risk of Glass Bursting: Boiling milk or boiling water reaches 100°C or higher. When exposed to this temperature, mercury expands violently beyond the top of the capillary bore, causing the glass bulb and stem to shatter explosively, creating severe burn hazards and toxic mercury vapor exposure.
CBSE Marking Scheme 2026-27
Limited scale range of clinical thermometer (35°C to 42°C)1 Mark
Severe risk of glass shattering due to excessive mercury thermal expansion1 Mark
Page 38 — Intext Q3 (Boojho's Query)
Boojho wondered why the level of mercury should change at all when a laboratory thermometer is taken out of water, while a clinical thermometer retains its reading.
2 Marks
✅ CBSE Standard Answer
Key Structural Difference:
1. Presence of a Kink in Clinical Thermometer: A clinical thermometer has a sharp narrow constriction or kink in the capillary tube just above the mercury bulb. When removed from the mouth or armpit, the cooler room air causes mercury in the bulb to contract, but the kink physically blocks the mercury column from flowing back, locking the reading in place until shaken.
2. Absence of Kink in Laboratory Thermometer: A laboratory thermometer has a completely straight, uniform capillary bore without any constriction. As soon as it is removed from hot water, the surrounding cooler air causes immediate contraction of mercury, making the level drop instantly. Hence, laboratory thermometers must be read while immersed in the substance.
1. Presence of a Kink in Clinical Thermometer: A clinical thermometer has a sharp narrow constriction or kink in the capillary tube just above the mercury bulb. When removed from the mouth or armpit, the cooler room air causes mercury in the bulb to contract, but the kink physically blocks the mercury column from flowing back, locking the reading in place until shaken.
2. Absence of Kink in Laboratory Thermometer: A laboratory thermometer has a completely straight, uniform capillary bore without any constriction. As soon as it is removed from hot water, the surrounding cooler air causes immediate contraction of mercury, making the level drop instantly. Hence, laboratory thermometers must be read while immersed in the substance.
CBSE Marking Scheme 2026-27
Presence of a constriction/kink in clinical thermometer preventing backflow1 Mark
Straight bore in lab thermometer causing immediate mercury drop on exposure to air1 Mark
NCERT Textbook Exercise Questions
Q1
State similarities and differences between the laboratory thermometer and the clinical thermometer.
3 Marks
✅ CBSE Standard Answer
Similarities:
- Both thermometers consist of a long, narrow, uniform glass capillary tube with a sealed liquid bulb at one end.
- Both traditionally use mercury (or coloured alcohol) as the thermometric liquid that expands uniformly upon heating.
- Both are graduated with the Celsius scale (°C) to measure temperature.
Differences:
| Parameter | Clinical Thermometer | Laboratory Thermometer |
|---|---|---|
| Measurement Range | 35°C to 42°C (spans human body temperature variation). | -10°C to 110°C (wide range for chemical experiments). |
| Constriction (Kink) | Has a kink in the capillary tube near the bulb to prevent mercury from backflowing. | Has no kink; mercury level falls immediately when removed from the medium. |
| Taking Readings | Reading can be taken conveniently after removing it from the patient's mouth or armpit. | Reading must be recorded while the thermometer bulb remains immersed in the liquid. |
| Jerking | Must be given a few sharp jerks to reset mercury below 35°C before reuse. | Does not require jerking; resets spontaneously to ambient temperature. |
CBSE Marking Scheme 2026-27
Listing at least two valid similarities (glass tube, mercury, °C scale)1 Mark
Differences in range (35–42°C vs -10 to 110°C) and presence/absence of kink2 Marks
Q2
Give two examples each of conductors and insulators of heat.
2 Marks
✅ CBSE Standard Answer
1. Conductors of Heat (Materials that permit thermal energy to pass easily):
- Copper (used in bottom of cookware and heat exchangers)
- Aluminium / Iron (used in cooking utensils, radiators)
2. Insulators of Heat (Poor conductors that impede heat transfer):
- Wood (used for handles of cooking pots)
- Plastic / Bakelite (used for electrical iron and pressure cooker handles)
- (Also Air, Glass, Wool, Thermocol).
CBSE Marking Scheme 2026-27
Two correct conductors with definition1 Mark
Two correct insulators with definition1 Mark
Q3
Fill in the blanks:
(a) The hotness of an object is determined by its _________.
(b) Temperature of boiling water cannot be measured by a _________ thermometer.
(c) Temperature is measured in degree _________.
(d) No medium is required for transfer of heat by the process of _________.
(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of _________.
(f) Clothes of _________ colours absorb heat better than clothes of light colours.
(a) The hotness of an object is determined by its _________.
(b) Temperature of boiling water cannot be measured by a _________ thermometer.
(c) Temperature is measured in degree _________.
(d) No medium is required for transfer of heat by the process of _________.
(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of _________.
(f) Clothes of _________ colours absorb heat better than clothes of light colours.
3 Marks
✅ CBSE Standard Answer
(a) The hotness of an object is determined by its temperature.
(b) Temperature of boiling water cannot be measured by a clinical thermometer. (Since clinical range ends at 42°C while boiling water is 100°C; it would burst).
(c) Temperature is measured in degree Celsius (°C).
(d) No medium is required for transfer of heat by the process of radiation.
(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.
(f) Clothes of dark colours absorb heat better than clothes of light colours.
CBSE Marking Scheme 2026-27
Each correct fill-in-the-blank (0.5 Mark × 6)3 Marks
Q4
Match the following:
(i) Land breeze blows during — (a) summer
(ii) Sea breeze blows during — (b) winter
(iii) Dark coloured clothes are preferred during — (c) day
(iv) Light coloured clothes are preferred during — (d) night
(i) Land breeze blows during — (a) summer
(ii) Sea breeze blows during — (b) winter
(iii) Dark coloured clothes are preferred during — (c) day
(iv) Light coloured clothes are preferred during — (d) night
2 Marks
✅ CBSE Standard Answer
| Phenomenon / Practice | Correct Matching | Scientific Reason |
|---|---|---|
| (i) Land breeze blows during | (d) night | Land cools down faster than sea water at night; cooler air from land moves towards sea. |
| (ii) Sea breeze blows during | (c) day | Land heats up faster than sea during day; warm air rises, cool sea air rushes in. |
| (iii) Dark coloured clothes are preferred during | (b) winter | Dark fabrics absorb maximum radiant heat from sunlight, keeping body warm. |
| (iv) Light coloured clothes are preferred during | (a) summer | Light fabrics reflect most of the incident solar radiation, keeping body cool. |
CBSE Marking Scheme 2026-27
Each correct match with brief rationale (0.5 Mark × 4)2 Marks
Q5
Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
2 Marks
✅ CBSE Standard Answer
Wearing multiple layers of clothing keeps us significantly warmer than wearing one thick garment due to the insulating properties of trapped air:
1. Trapped Air Layers: When we wear multiple thin layers of clothes, pockets of air become trapped in the microscopic spaces between adjacent layers of fabric.
2. Air is a Poor Conductor of Heat: Air is one of the poorest conductors (best insulators) of heat in nature. The motionless trapped air forms an effective thermal barrier that prevents body heat from conducting outwards into the freezing ambient surroundings.
Conclusion: Multiple trapped air interfaces provide far superior thermal resistance compared to a single thick piece of fabric, which has only two exterior air-contact surfaces.
CBSE Marking Scheme 2026-27
Mention of air trapped between multiple layers of fabric1 Mark
Air acting as a poor conductor/insulator preventing body heat loss1 Mark
Q6
Look at a vessel containing water boiling over a burner flame. Mark where the heat is being transferred by conduction, by convection and by radiation.
3 Marks
✅ CBSE Standard Answer
1. Conduction: Takes place through the solid metallic base and sides of the pan. Heat transfers from the outer bottom surface of the metal pan directly to its inner surface in contact with water, and along the pan handle.
2. Convection: Takes place within the water inside the pan. The bottom water layer heats up, expands, becomes lighter, and rises upwards. Cold, denser water from the top sinks to the bottom, creating circular convection currents that heat the entire body of water.
3. Radiation: Takes place through the air surrounding the flame and hot pan. Thermal infrared waves travel directly outward in all directions from the burner flame and hot outer pan surfaces without needing physical contact.
CBSE Marking Scheme 2026-27
Conduction through solid metal pan base/handle1 Mark
Convection within water via convection currents1 Mark
Radiation through air from flame and exterior pan1 Mark
Q7
In places of hot climate, it is advised that the outer walls of houses be painted white. Explain.
2 Marks
✅ CBSE Standard Answer
1. High Reflection: White and light-coloured surfaces are excellent reflectors of radiant heat. They reflect back most of the intense sunlight falling upon them.
2. Poor Absorption: White surfaces absorb a minimal fraction of solar infrared radiation compared to dark surfaces.
Conclusion: As a result, the outer walls absorb very little heat during scorching daytime hours, keeping the interior of the house substantially cooler, reducing room temperature and cooling costs.
CBSE Marking Scheme 2026-27
White surfaces reflect most incident solar radiant heat1 Mark
Minimal heat absorption keeps building interiors cooler1 Mark
Q8
One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be: (a) 80°C (b) more than 50°C but less than 80°C (c) 20°C (d) between 30°C and 50°C
1 Mark
✅ CBSE Standard Answer
Correct Option: (d) between 30°C and 50°C
Scientific Principle of Heat Exchange: Heat flows spontaneously from a warmer body to a cooler body until thermal equilibrium is attained:
Heat Lost by Hot Water = Heat Gained by Cold Water
For equal masses of water: T_final = (30°C + 50°C) / 2 = 80 / 2 = 40°C. Since 40°C lies strictly between 30°C and 50°C, option (d) is correct.
Heat Lost by Hot Water = Heat Gained by Cold Water
For equal masses of water: T_final = (30°C + 50°C) / 2 = 80 / 2 = 40°C. Since 40°C lies strictly between 30°C and 50°C, option (d) is correct.
CBSE Marking Scheme 2026-27
Option (d) with heat equilibrium explanation1 Mark
Q9
An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will: (a) flow from iron ball to water (b) not flow from iron ball to water or from water to iron ball (c) flow from water to iron ball (d) increase the temperature of both
1 Mark
✅ CBSE Standard Answer
Correct Option: (b) not flow from iron ball to water or from water to iron ball
Scientific Reason: The flow of thermal energy requires a temperature difference (ΔT > 0) between two contacting bodies. Since both the iron ball and water are at the exact same temperature (40°C), they are in a state of thermal equilibrium. Hence, no net heat transfer takes place between them.
CBSE Marking Scheme 2026-27
Option (b) with thermal equilibrium principle1 Mark
Q10
A wooden spoon is dipped in a cup of ice cream. Its other end: (a) becomes cold by the process of conduction (b) becomes cold by the process of convection (c) becomes cold by the process of radiation (d) does not become cold
1 Mark
✅ CBSE Standard Answer
Correct Option: (d) does not become cold
Scientific Reason: Wood is a non-metal and an insulator (poor conductor of heat). It does not allow thermal energy to conduct from one end of the spoon to the other. Therefore, the outer handle does not become cold.
CBSE Marking Scheme 2026-27
Option (d) with wood insulator explanation1 Mark
Q11
Stainless steel pans are usually provided with copper bottoms. The reason for this could be that: (a) copper bottom makes the pan more durable (b) such pans appear colourful (c) copper is a better conductor of heat than the stainless steel (d) copper is easier to clean than the stainless steel
1 Mark
✅ CBSE Standard Answer
Correct Option: (c) copper is a better conductor of heat than the stainless steel
Scientific Reason: Copper has significantly higher thermal conductivity than stainless steel alloy. A copper base conducts heat from the stove burner much faster and distributes heat evenly across the base of the pan, cooking food quickly while preventing localized burning (hotspots).
CBSE Marking Scheme 2026-27
Option (c) with thermal conductivity comparison1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Coastal Breeze Convection Cycles
Scenario: Coastal cities like Mumbai and Chennai experience pleasant breezes during both day and night without drastic diurnal temperature extremes seen in desert cities like Jaisalmer.
(i) What is Sea Breeze? Explain its formation mechanism during daytime.
(ii) What is Land Breeze? Explain why it reverses during nighttime.
(iii) Why do windows of houses in coastal areas ideally face the sea?
(i) What is Sea Breeze? Explain its formation mechanism during daytime.
(ii) What is Land Breeze? Explain why it reverses during nighttime.
(iii) Why do windows of houses in coastal areas ideally face the sea?
(i) Sea Breeze (Daytime): Land heats up much faster than water due to its lower specific heat capacity. During the day, air above land heats up, expands, and rises. Cool, dense air from over the sea moves inland towards land to take its place. This cool onshore wind is the Sea Breeze.
(ii) Land Breeze (Nighttime): At night, land cools down much faster than sea water. The sea water remains relatively warm, causing the air above it to rise. Cooler, denser air from over the land blows seaward to replace it. This offshore wind is the Land Breeze.
(iii) Architectural Design: Windows are built facing the sea to directly receive the continuous, refreshing daytime sea breeze, naturally cooling the interior without requiring artificial air conditioning.
⚡ Assertion & Reason (HOTS)
Assertion (A): A clinical thermometer should never be sterilised by dipping in boiling water.
Reason (R): The maximum calibrated range of a clinical thermometer is 42°C, whereas boiling water reaches 100°C, causing extreme mercury expansion that can shatter the glass capillary.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): The maximum calibrated range of a clinical thermometer is 42°C, whereas boiling water reaches 100°C, causing extreme mercury expansion that can shatter the glass capillary.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Clinical thermometers are specially engineered only for human physiological range (35°C–42°C). Dipping in boiling water (100°C) produces excessive hydrostatic pressure from expanding mercury, which will break the sealed glass tube and release toxic mercury vapor. Antiseptic solutions (like Dettol or alcohol wipes) should be used instead.
4
Chapter 4: Acids, Bases and Salts
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Neutralisation Reactions
- Acids: Substances with a sour taste, turn blue litmus red, pH < 7. Contain hydrogen ions (H⁺).
- Natural/Organic Acids: Acetic acid (Vinegar), Formic acid (Ant sting), Citric acid (Lemons, Oranges), Lactic acid (Curd), Oxalic acid (Spinach), Ascorbic acid/Vitamin C (Amla, Citrus), Tartaric acid (Tamarind, Grapes).
- Mineral Acids: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃).
- Bases: Substances bitter in taste and soapy/slippery to touch, turn red litmus blue, pH > 7. Contain hydroxide ions (OH⁻).
- Calcium hydroxide (Lime water), Ammonium hydroxide (Window cleaner), Sodium hydroxide / Potassium hydroxide (Soap), Magnesium hydroxide (Milk of Magnesia).
- Indicators: Special chemical substances that exhibit distinct colour changes in acidic and basic solutions:
- Litmus (from Lichens): Mauve/purple in neutral; Red in acid; Blue in base.
- Turmeric: Yellow in acid/neutral; turns Reddish-brown in base.
- China Rose Petal Solution: Turns dark pink (magenta) in acid; green in base.
- Phenolphthalein: Colourless in acid; turns pink in base.
- Neutralisation Reaction: The reaction between an acid and a base producing a salt and water with the evolution of heat:
Acid + Base ⎯⎯→ Salt + Water + Heat
HCl (Hydrochloric acid) + NaOH (Sodium hydroxide) ⎯⎯→ NaCl (Sodium chloride) + H₂O + Heat
NCERT Intext Questions & Scientific Enquiries
Page 49 — Intext Q1 (Boojho's Caution)
Boojho asks: 'Can I taste all unknown substances in the laboratory to determine whether they are acidic or basic?' What warning does Paheli give him?
2 Marks
✅ CBSE Standard Answer
Answer: No, we must NEVER taste unknown chemicals or substances.
Paheli's Warning & Scientific Justification:
1. Severe Toxicity & Corrosiveness: Many laboratory acids (like concentrated sulfuric acid, nitric acid, hydrochloric acid) and strong bases (like sodium hydroxide, potassium hydroxide) are extremely corrosive and toxic. Even a microscopic droplet can burn mouth tissues, destroy mucous membranes, or cause fatal chemical poisoning.
2. Safe Scientific Testing: Scientists use safe, specialized chemical and natural indicators (such as litmus paper, turmeric paper, phenolphthalein, and China rose petals) to test whether an unknown substance is acidic or basic without tasting or touching.
1. Severe Toxicity & Corrosiveness: Many laboratory acids (like concentrated sulfuric acid, nitric acid, hydrochloric acid) and strong bases (like sodium hydroxide, potassium hydroxide) are extremely corrosive and toxic. Even a microscopic droplet can burn mouth tissues, destroy mucous membranes, or cause fatal chemical poisoning.
2. Safe Scientific Testing: Scientists use safe, specialized chemical and natural indicators (such as litmus paper, turmeric paper, phenolphthalein, and China rose petals) to test whether an unknown substance is acidic or basic without tasting or touching.
CBSE Marking Scheme 2026-27
Strict prohibition due to corrosive and toxic nature of chemicals1 Mark
Use of indicators as safe scientific method of testing1 Mark
Page 50 — Intext Q2 (Paheli's Query)
Paheli noticed that a yellow curry (turmeric) stain on her white shirt turns reddish-brown when washed with bathing soap. Why does this happen, and why does it turn yellow again on rinsing with plenty of water?
2 Marks
✅ CBSE Standard Answer
Scientific Explanation:
1. Turmeric is a Natural Indicator: Turmeric contains the natural pigment curcumin, which remains yellow in acidic and neutral environments but turns distinctive reddish-brown in basic (alkaline) solutions.
2. Soap is Basic: Laundry and bathing soaps contain sodium or potassium salts of fatty acids, which are alkaline (pH > 7). When soap is rubbed onto the curry stain, the base reacts with turmeric, shifting its color to reddish-brown.
3. Restoration to Yellow: When the shirt is thoroughly rinsed with copious clean water (or treated with lemon juice), the alkaline soap is washed away, restoring the neutral/acidic condition, and the stain reverts to its original yellow color.
1. Turmeric is a Natural Indicator: Turmeric contains the natural pigment curcumin, which remains yellow in acidic and neutral environments but turns distinctive reddish-brown in basic (alkaline) solutions.
2. Soap is Basic: Laundry and bathing soaps contain sodium or potassium salts of fatty acids, which are alkaline (pH > 7). When soap is rubbed onto the curry stain, the base reacts with turmeric, shifting its color to reddish-brown.
3. Restoration to Yellow: When the shirt is thoroughly rinsed with copious clean water (or treated with lemon juice), the alkaline soap is washed away, restoring the neutral/acidic condition, and the stain reverts to its original yellow color.
CBSE Marking Scheme 2026-27
Turmeric acting as natural indicator turning reddish-brown with basic soap1 Mark
Rinsing with water removing alkaline base, restoring yellow neutral color1 Mark
NCERT Textbook Exercise Questions
Q1
State differences between acids and bases.
3 Marks
✅ CBSE Standard Answer
| Property | Acids | Bases |
|---|---|---|
| Taste | Have a distinctly sour taste (e.g., lemon juice, vinegar). | Have a distinctly bitter taste (e.g., baking soda, neem). |
| Touch / Texture | Do not feel soapy; can cause burning sensations when concentrated. | Feel soapy and slippery to the touch. |
| Action on Litmus | Turn blue litmus paper red. | Turn red litmus paper blue. |
| Phenolphthalein Indicator | Remains colourless. | Turns bright pink. |
| Turmeric Paper | Remains yellow (no colour change). | Turns reddish-brown. |
| Examples | Citric acid, hydrochloric acid (HCl), acetic acid. | Sodium hydroxide (NaOH), calcium hydroxide (slaked lime). |
CBSE Marking Scheme 2026-27
Taste and touch distinctions1 Mark
Action on litmus, phenolphthalein and turmeric indicators1 Mark
Chemical examples for both classes1 Mark
Q2
Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
1 Mark
✅ CBSE Standard Answer
Nature of Ammonia: Basic (Alkaline).
Scientific Reason: Bases have the characteristic chemical property of turning red litmus paper blue. Ammonia dissolves in water to form ammonium hydroxide (NH₄OH), which releases hydroxide ions (OH⁻), rendering the cleaner alkaline.
CBSE Marking Scheme 2026-27
Stating basic nature + litmus test rationale1 Mark
Q3
Name the source from which litmus solution is obtained. What is the use of this solution?
2 Marks
✅ CBSE Standard Answer
Source: Natural litmus dye solution is extracted from Lichens (a composite symbiotic organism of an alga and a fungus). In distilled neutral water, it possesses a characteristic mauve (purple) colour.
Use: Litmus solution is used universally as an acid-base indicator to test the chemical nature of unknown aqueous solutions:
- When added to an acidic solution, it turns red.
- When added to a basic solution, it turns blue.
- In a neutral solution, it remains purple.
CBSE Marking Scheme 2026-27
Source identified as Lichens1 Mark
Use as indicator with colour shifts in acid (red) and base (blue)1 Mark
Q4
Is the distilled water acidic/basic/neutral? How would you verify it?
2 Marks
✅ CBSE Standard Answer
Nature of Distilled Water: Distilled water is completely Neutral (pH = 7).
Experimental Verification:
- Take two strips of litmus paper: one red litmus paper and one blue litmus paper.
- Place a few drops of pure distilled water onto both strips using a clean dropper.
- Observation:
- Red litmus paper does not change colour (remains red).
- Blue litmus paper does not change colour (remains blue).
- Conclusion: Since the water affects neither blue nor red litmus, it is neither acidic nor basic; it is confirmed to be strictly neutral.
CBSE Marking Scheme 2026-27
State neutral nature0.5 Mark
Verification test using both red and blue litmus strips1.5 Marks
Q5
Describe the process of neutralisation with the help of an example.
3 Marks
✅ CBSE Standard Answer
Definition: Neutralisation is the chemical reaction between an acid and a base. The acidic nature of the acid and the alkaline nature of the base destroy each other, resulting in the formation of a neutral salt and water along with the release of thermal energy:
Acid + Base ⎯⎯→ Salt + Water + Heat
Acid + Base ⎯⎯→ Salt + Water + Heat
Experimental Example:
- Take 5 mL of dilute Hydrochloric acid (HCl) in a conical flask and add 2–3 drops of phenolphthalein indicator. The solution remains colourless.
- Using a burette, add dilute Sodium hydroxide solution (NaOH) drop by drop with continuous swirling.
- At the exact neutralisation point, the solution turns a faint persistent pink.
- Now add one more drop of dilute HCl; the pink colour vanishes. Add a drop of NaOH; pink colour returns.
- Touching the flask reveals that the mixture has become warm, showing that heat is evolved.
Chemical Equation:
HCl (Hydrochloric acid) + NaOH (Sodium hydroxide) ⎯⎯→ NaCl (Sodium chloride) + H₂O (Water) + Heat
HCl (Hydrochloric acid) + NaOH (Sodium hydroxide) ⎯⎯→ NaCl (Sodium chloride) + H₂O (Water) + Heat
CBSE Marking Scheme 2026-27
Accurate definition with word equation1 Mark
Detailed experimental demonstration using phenolphthalein1 Mark
Balanced chemical equation (HCl + NaOH → NaCl + H₂O)1 Mark
Q6
Mark 'T' if the statement is true and 'F' if it is false:
(a) Nitric acid turns red litmus blue. (T/F)
(b) Sodium hydroxide turns blue litmus red. (T/F)
(c) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
(d) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
(e) Tooth decay is caused by the presence of a base. (T/F)
(a) Nitric acid turns red litmus blue. (T/F)
(b) Sodium hydroxide turns blue litmus red. (T/F)
(c) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
(d) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
(e) Tooth decay is caused by the presence of a base. (T/F)
2.5 Marks
✅ CBSE Standard Answer
(a) False (F) — Nitric acid is a strong acid; it turns blue litmus red, not red litmus blue.
(b) False (F) — Sodium hydroxide is a strong base; it turns red litmus blue.
(c) True (T) — This is a classical neutralisation reaction forming sodium chloride (table salt) and water.
(d) True (T) — Chemical indicators exhibit characteristic colour transitions depending on hydrogen ion concentrations.
(e) False (F) — Tooth decay is caused by acids produced by mouth bacteria fermenting leftover sugar, corroding the enamel.
CBSE Marking Scheme 2026-27
Each correct True/False determination (0.5 Mark × 5)2.5 Marks
Q7
Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?
3 Marks
✅ CBSE Standard Answer
Dorji can easily distinguish the three drinks by testing a drop from each unlabelled bottle using red and blue litmus papers (or by tasting a drop, since edible soft drinks are safe):
1. Identification of the Acidic Drink:
Dip a strip of blue litmus paper into a small sample of each drink. The drink that turns blue litmus paper RED is the acidic drink (serve to customer 1).
Dip a strip of blue litmus paper into a small sample of each drink. The drink that turns blue litmus paper RED is the acidic drink (serve to customer 1).
2. Identification of the Basic Drink:
Dip a strip of red litmus paper into a sample of the remaining drinks. The drink that turns red litmus paper BLUE is the basic drink (serve to customer 2).
Dip a strip of red litmus paper into a sample of the remaining drinks. The drink that turns red litmus paper BLUE is the basic drink (serve to customer 2).
3. Identification of the Neutral Drink:
The drink that causes no colour change on either red or blue litmus paper is the neutral drink (serve to customer 3).
The drink that causes no colour change on either red or blue litmus paper is the neutral drink (serve to customer 3).
CBSE Marking Scheme 2026-27
Testing for acidic drink (blue litmus turns red)1 Mark
Testing for basic drink (red litmus turns blue)1 Mark
Testing for neutral drink (no colour change on either litmus)1 Mark
Q8
Explain why:
(a) An antacid tablet is taken when you suffer from acidity.
(b) Calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralised before disposing it into the water bodies.
(a) An antacid tablet is taken when you suffer from acidity.
(b) Calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralised before disposing it into the water bodies.
3 Marks
✅ CBSE Standard Answer
(a) Antacid for Acidity: The stomach naturally secretes hydrochloric acid (HCl) to digest food. During indigestion or hyperacidity, excess HCl causes irritation, pain, and burning sensations. Antacid tablets contain a mild, non-toxic base such as Milk of Magnesia (Magnesium hydroxide) or aluminium hydroxide. The base neutralises the excess stomach acid, forming salt and water, providing rapid relief:
2HCl + Mg(OH)₂ ⎯⎯→ MgCl₂ + 2H₂O
2HCl + Mg(OH)₂ ⎯⎯→ MgCl₂ + 2H₂O
(b) Calamine on Ant Bite: When an ant stings, it injects acidic Formic acid (methanoic acid) into the dermal skin layers, causing burning pain, inflammation, and itching. Calamine solution contains zinc carbonate (ZnCO₃), which is a mild base. Applying calamine neutralises the acidic formic acid, relieving swelling and pain immediately.
(c) Neutralisation of Factory Waste: Industrial effluents from chemical and manufacturing plants often contain high concentrations of toxic acids or strong alkalis. If discharged directly into rivers, lakes, or oceans, the sharp change in aquatic pH will kill fish, aquatic plants, and beneficial microorganisms. Hence, factories neutralise acidic wastes by adding basic substances (like slaked lime) before safe discharge.
CBSE Marking Scheme 2026-27
Part (a): Neutralisation of excess HCl by magnesium hydroxide1 Mark
Part (b): Neutralisation of formic acid by zinc carbonate in calamine1 Mark
Part (c): Preventing death of aquatic life from pH shock1 Mark
Q9
Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them using only turmeric indicator?
3 Marks
✅ CBSE Standard Answer
Turmeric paper is naturally bright yellow. It remains yellow in acidic and neutral solutions, but turns reddish-brown in basic solutions. We can identify all three liquids systematically:
Step 1: Identifying Sodium Hydroxide (Base):
Place a drop of each of the three liquids on three separate strips of yellow turmeric paper.
The liquid that turns the yellow turmeric paper REDDISH-BROWN is Sodium hydroxide (NaOH).
Place a drop of each of the three liquids on three separate strips of yellow turmeric paper.
The liquid that turns the yellow turmeric paper REDDISH-BROWN is Sodium hydroxide (NaOH).
Step 2: Differentiating Acid (HCl) and Neutral (Sugar Solution):
Take two separate portions of the reddish-brown turmeric paper (or red-turned paste) created in Step 1:
• Add a few drops of the second liquid to one red strip, and the third liquid to the other.
• The liquid that neutralises the base and turns the reddish-brown paper back to YELLOW is Hydrochloric acid (HCl).
• The remaining liquid that causes no colour change on the red paper is the Sugar solution (Neutral).
Take two separate portions of the reddish-brown turmeric paper (or red-turned paste) created in Step 1:
• Add a few drops of the second liquid to one red strip, and the third liquid to the other.
• The liquid that neutralises the base and turns the reddish-brown paper back to YELLOW is Hydrochloric acid (HCl).
• The remaining liquid that causes no colour change on the red paper is the Sugar solution (Neutral).
CBSE Marking Scheme 2026-27
Identification of NaOH (turns turmeric yellow to reddish-brown)1 Mark
Identification of HCl (reverses red turmeric back to yellow via neutralisation)1 Mark
Identification of sugar solution as remaining neutral liquid1 Mark
Q10
Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
2 Marks
✅ CBSE Standard Answer
The solution can be either Basic or Neutral.
Scientific Explanation:
- Possibility 1 (Basic Solution): Bases turn red litmus blue and have no effect on blue litmus paper. Therefore, a basic solution will leave blue litmus paper blue.
- Possibility 2 (Neutral Solution): Neutral substances (such as pure water, salt solution, sugar solution) cause no colour change on either red or blue litmus paper. Hence, blue litmus paper remains blue in a neutral solution as well.
CBSE Marking Scheme 2026-27
Stating both possibilities: Basic or Neutral1 Mark
Scientific explanation for both cases + confirmatory test1 Mark
Q11
Consider the following statements:
(a) Both acids and bases change colour of all indicators.
(b) If an indicator gives a colour change with an acid, it does not give a change with a base.
(c) If an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Which of these statements are correct? (i) All four (ii) a and d (iii) b, c and d (iv) only d
(a) Both acids and bases change colour of all indicators.
(b) If an indicator gives a colour change with an acid, it does not give a change with a base.
(c) If an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Which of these statements are correct? (i) All four (ii) a and d (iii) b, c and d (iv) only d
1 Mark
✅ CBSE Standard Answer
Correct Option: (iv) only d
Scientific Explanation: Different indicators respond distinctively depending on their unique molecular composition. For instance, litmus changes colour in both acid (red) and base (blue), while turmeric only changes colour in a base (turns red) and remains yellow in acid. Phenolphthalein is colourless in acid and pink in base. Thus, the specific colour change depends entirely on the type and chemical nature of the indicator used.
CBSE Marking Scheme 2026-27
Option (iv) only d with explanation1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Soil Treatment in Agriculture
Agricultural Case: A soil testing lab analyzed samples from two different agricultural fields:
• Field A: Overused with excessive chemical nitrogenous fertilisers; tested highly acidic (pH 4.8). Crop roots are decaying.
• Field B: Located near limestone quarries; tested excessively alkaline (pH 8.9). Nutrients are insoluble.
(i) What should the farmer add to Field A to treat the soil? Explain the chemical principle.
(ii) What should be added to Field B to remediate the alkaline soil?
(iii) Why does extreme soil pH prevent healthy crop growth?
• Field A: Overused with excessive chemical nitrogenous fertilisers; tested highly acidic (pH 4.8). Crop roots are decaying.
• Field B: Located near limestone quarries; tested excessively alkaline (pH 8.9). Nutrients are insoluble.
(i) What should the farmer add to Field A to treat the soil? Explain the chemical principle.
(ii) What should be added to Field B to remediate the alkaline soil?
(iii) Why does extreme soil pH prevent healthy crop growth?
(i) Treatment for Field A (Acidic Soil): The farmer should treat the soil with bases such as quicklime (calcium oxide, CaO) or slaked lime (calcium hydroxide, Ca(OH)₂). The alkaline lime chemically neutralises the excessive soil acidity, raising the pH to an optimal neutral level (~6.5–7).
(ii) Treatment for Field B (Basic Soil): The farmer should add organic matter (manure or compost). As organic manure decays, soil microorganisms release organic acids that neutralise the excessive alkaline bases in the soil.
(iii) Biological Reason: Plants cannot absorb essential minerals (nitrogen, phosphorus, potassium) efficiently when soil is either too acidic or too basic, leading to stunted growth and wilting.
⚡ Assertion & Reason (HOTS)
Assertion (A): When soap is rubbed onto a yellow curry stain on a white shirt, the stain turns reddish-brown.
Reason (R): Soap solution contains bases (sodium/potassium salts of fatty acids), which react with the natural turmeric indicator present in the curry stain to produce a reddish-brown colour.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Soap solution contains bases (sodium/potassium salts of fatty acids), which react with the natural turmeric indicator present in the curry stain to produce a reddish-brown colour.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Curry contains turmeric, a natural pH indicator. Turmeric contains curcumin, which remains yellow in acidic or neutral mediums but turns reddish-brown at alkaline pH (above 7.5). Washing soap is basic, causing the instantaneous colour change. When rinsed with plenty of acidic lemon water, the yellow color returns.
5
Chapter 5: Physical and Chemical Changes
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Chemical Reactions
- Physical Changes: Changes in which only the physical properties (shape, size, state, appearance) of a substance undergo transformation, with no new chemical substance formed. Generally temporary and easily reversible. E.g., melting of ice, boiling of water, tearing of paper, dissolution of sugar in water.
- Chemical Changes (Chemical Reactions): Changes in which one or more entirely new substances with distinct chemical compositions and properties are formed. Irreversible and accompanied by energy changes (heat, light, sound), evolution of gas, change in colour, or precipitate formation. E.g., burning of fuels, rusting of iron, curdling of milk, digestion of food.
- Classic Laboratory Chemical Reactions:
- Burning of Magnesium Ribbon: Burns with a dazzling white flame:
2Mg (Magnesium) + O₂ (Oxygen) ⎯⎯→ 2MgO (Magnesium oxide - basic ash)
MgO + H₂O ⎯⎯→ Mg(OH)₂ (Magnesium hydroxide — turns red litmus blue) - Copper Sulphate and Iron Nail Reaction (Displacement):
CuSO₄ (Blue solution) + Fe (Iron nail) ⎯⎯→ FeSO₄ (Light green solution) + Cu (Brown deposit on nail) - Baking Soda and Vinegar Reaction:
CH₃COOH (Vinegar / Acetic acid) + NaHCO₃ (Baking soda) ⎯⎯→ CO₂ (Carbon dioxide) + H₂O + Sodium acetate
CO₂ + Ca(OH)₂ (Lime water) ⎯⎯→ CaCO₃ (Insoluble Calcium carbonate, milky) + H₂O
- Burning of Magnesium Ribbon: Burns with a dazzling white flame:
- Rusting of Iron: Corrosion of iron in the simultaneous presence of both oxygen (air) and moisture (water):
4Fe + 3O₂ + 2xH₂O ⎯⎯→ 2Fe₂O₃·xH₂O (Hydrated iron(III) oxide / Rust)
Prevention: Barrier protection (painting, greasing, oiling), alloying (stainless steel), and galvanisation (coating iron with a sacrificial protective layer of molten zinc metal). - Crystallisation: A physical purification technique in which pure, well-defined solid crystals of a substance are separated from its hot, saturated solution upon cooling.
NCERT Intext Questions & Scientific Enquiries
Page 58 — Intext Q1
Paheli asks: 'When we tear a large sheet of paper into small pieces, we cannot join the pieces back to get the original sheet. Then why is tearing paper classified as a physical change?'
2 Marks
✅ CBSE Standard Answer
Definition of Physical Change:
1. A change is classified as physical if only the physical properties (size, shape, texture, state) of a substance are altered, with no new chemical substance being formed.
2. When paper is torn, each small bit of paper is still made of the exact same chemical cellulose molecules as the original sheet.
3. Even though the mechanical process is physically irreversible (we cannot un-tear paper), no chemical bonds were broken or formed, and no new chemical species emerged. Therefore, tearing paper is purely a physical change.
1. A change is classified as physical if only the physical properties (size, shape, texture, state) of a substance are altered, with no new chemical substance being formed.
2. When paper is torn, each small bit of paper is still made of the exact same chemical cellulose molecules as the original sheet.
3. Even though the mechanical process is physically irreversible (we cannot un-tear paper), no chemical bonds were broken or formed, and no new chemical species emerged. Therefore, tearing paper is purely a physical change.
CBSE Marking Scheme 2026-27
Definition of physical change: alteration of size/shape with no new substance1 Mark
Chemical composition of cellulose remaining unchanged despite irreversibility1 Mark
Page 62 — Intext Q2 (Boojho's Query)
Boojho asks: 'Why does passing carbon dioxide gas through freshly prepared lime water turn it milky? Give the chemical equation.'
2 Marks
✅ CBSE Standard Answer
Chemical Reaction & Explanation:
1. Fresh lime water is a clear, aqueous solution of calcium hydroxide: Ca(OH)₂.
2. When carbon dioxide gas (CO₂) is bubbled through lime water, it reacts chemically to form insoluble white precipitate particles of calcium carbonate (CaCO₃) suspended in water:
3. The formation of these insoluble white calcium carbonate particles scatters light, making the clear solution appear distinctly milky. This serves as the standard confirmatory test for carbon dioxide gas.
1. Fresh lime water is a clear, aqueous solution of calcium hydroxide: Ca(OH)₂.
2. When carbon dioxide gas (CO₂) is bubbled through lime water, it reacts chemically to form insoluble white precipitate particles of calcium carbonate (CaCO₃) suspended in water:
Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium carbonate (CaCO₃) [Milky ppt] + Water (H₂O)
3. The formation of these insoluble white calcium carbonate particles scatters light, making the clear solution appear distinctly milky. This serves as the standard confirmatory test for carbon dioxide gas.
CBSE Marking Scheme 2026-27
Chemical equation forming insoluble calcium carbonate (CaCO₃)1 Mark
Suspension of white CaCO₃ particles causing milky appearance1 Mark
NCERT Textbook Exercise Questions
Q1
Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f) Digestion of food
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f) Digestion of food
3 Marks
✅ CBSE Standard Answer
| Process | Type of Change | Scientific Rationale |
|---|---|---|
| (a) Photosynthesis | Chemical change | CO₂ and H₂O react in sunlight to form entirely new substances: glucose and oxygen. |
| (b) Dissolving sugar in water | Physical change | No new substance is formed; sugar can be recovered unchanged by evaporating the water. |
| (c) Burning of coal | Chemical change | Carbon reacts with atmospheric oxygen to form carbon dioxide, ash, and heat irreversibly. |
| (d) Melting of wax | Physical change | Solid wax changes state to liquid wax; on cooling, solid wax is reformed with no new substance. |
| (e) Beating aluminium to make foil | Physical change | Only shape and thickness of aluminium metal change (malleability); chemical identity remains intact. |
| (f) Digestion of food | Chemical change | Complex macromolecules (starch, proteins, fats) are enzymatically broken down into new simple compounds (glucose, amino acids). |
CBSE Marking Scheme 2026-27
Each correct classification with valid scientific reasoning (0.5 Mark × 6)3 Marks
Q2
State whether the following statements are true or false:
(a) Cutting a log of wood into pieces is a chemical change. (T/F)
(b) Formation of manure from leaves is a physical change. (T/F)
(c) Iron pipes coated with zinc do not get rusted easily. (T/F)
(d) Iron and rust are the same substance. (T/F)
(e) Condensation of steam is not a chemical change. (T/F)
(a) Cutting a log of wood into pieces is a chemical change. (T/F)
(b) Formation of manure from leaves is a physical change. (T/F)
(c) Iron pipes coated with zinc do not get rusted easily. (T/F)
(d) Iron and rust are the same substance. (T/F)
(e) Condensation of steam is not a chemical change. (T/F)
2.5 Marks
✅ CBSE Standard Answer
(a) False (F) — Cutting wood only alters its physical size and dimensions; no new chemical substance is formed (it remains wood). Hence it is a physical change.
(b) False (F) — Conversion of leaves into manure is biological decomposition by bacteria/fungi forming a new nutrient-rich substance (humus); it is a chemical change.
(c) True (T) — Coating iron with a layer of zinc (galvanisation) forms a protective barrier preventing air and moisture from contacting the iron.
(d) False (F) — Iron is a pure grey transition metal element (Fe), while rust is a reddish-brown chemical compound: hydrated iron(III) oxide (Fe₂O₃·xH₂O).
(e) True (T) — Condensation of steam into liquid water is merely a physical change of state from gas to liquid; water molecules (H₂O) remain unchanged.
CBSE Marking Scheme 2026-27
Each correct True/False answer with brief rationale (0.5 Mark × 5)2.5 Marks
Q3
Fill in the blanks:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.
(b) The chemical name of baking soda is _________.
(c) Two methods by which rusting of iron can be prevented are _________ and _________.
(d) Changes in which only _________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _________ changes.
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.
(b) The chemical name of baking soda is _________.
(c) Two methods by which rusting of iron can be prevented are _________ and _________.
(d) Changes in which only _________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _________ changes.
2.5 Marks
✅ CBSE Standard Answer
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate (CaCO₃).
(b) The chemical name of baking soda is sodium hydrogen carbonate (or sodium bicarbonate, NaHCO₃).
(c) Two methods by which rusting of iron can be prevented are painting/greasing and galvanisation.
(d) Changes in which only physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called chemical changes.
CBSE Marking Scheme 2026-27
Each correct blank accurately filled (0.5 Mark × 5)2.5 Marks
Q4
When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
2 Marks
✅ CBSE Standard Answer
Type of Change: It is a Chemical change.
Scientific Explanation:
- When lemon juice (which contains citric acid) reacts with baking soda (sodium hydrogen carbonate), a brisk effervescence occurs, releasing a new gas: carbon dioxide (CO₂) along with water and sodium citrate salt.
- Since entirely new chemical substances with distinct properties are produced that cannot be converted back by simple physical means, the reaction is chemical:
Lemon juice (Citric acid) + Baking soda (Sodium hydrogen carbonate) ⎯⎯→ Carbon dioxide (bubbles) + Sodium citrate + Water
CBSE Marking Scheme 2026-27
Identification as a chemical change1 Mark
Formation of new substance (carbon dioxide gas) with word equation1 Mark
Q5
When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both physical and chemical changes take place.
3 Marks
✅ CBSE Standard Answer
1. In Burning of a Candle:
- Physical Change: The heat of the flame melts the solid wax near the wick into liquid wax. When it drips down and cools, it resolidifies into solid wax without changing its chemical composition. This change of state is a physical change.
- Chemical Change: Molten wax rises through the wick and vaporises; the wax vapour combusts in atmospheric oxygen producing heat, light, carbon dioxide (CO₂), water vapour (H₂O), and soot. These new substances cannot be converted back into wax; hence, it is a chemical change.
2. Another Example Exhibiting Both Changes:
- Digestion of Food in the Human Body:
- Physical Change: Mechanical mastication and grinding of food by teeth into smaller physical pieces in the mouth.
- Chemical Change: Enzymatic breakdown of complex starch into maltose by saliva and digestion of proteins into amino acids by gastric juices.
- (Alternative: LPG cylinder usage — physical conversion of liquid LPG into gas as pressure drops, followed by chemical combustion of gas into CO₂ and H₂O).
CBSE Marking Scheme 2026-27
Melting of wax identified as physical change1 Mark
Burning of wax vapour identified as chemical change1 Mark
Another valid example explained (e.g. food digestion or LPG)1 Mark
Q6
How would you show that setting of curd is a chemical change?
2 Marks
✅ CBSE Standard Answer
Setting of curd is proven to be a chemical change based on the following scientific criteria:
1. Formation of a New Substance: When a small amount of curd (inoculum) is added to warm milk, the bacterium Lactobacillus ferments lactose sugar present in milk into lactic acid. The acid coagulates the milk protein (casein), forming an entirely new chemical substance: curd.
2. Change in Properties: Curd has a distinctly sour taste, acidic pH, and a thick semi-solid texture completely different from sweet liquid milk.
3. Irreversibility: Once curd is formed, it cannot be reversed back into milk by any physical separation method.
CBSE Marking Scheme 2026-27
Conversion of lactose to lactic acid by Lactobacillus forming a new substance1 Mark
Irreversible nature and altered chemical/taste properties1 Mark
Q7
Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
3 Marks
✅ CBSE Standard Answer
1. Cutting of Wood (Physical Change):
- When a wooden log is sawn or chopped into smaller pieces, only its physical dimensions (size and shape) are altered.
- The chemical composition of each wood chip remains identical to the original log (cellulose and lignin).
- No new chemical substance is formed; therefore, cutting wood is purely a physical change.
2. Burning of Wood (Chemical Change):
- When wood is ignited, the carbon and organic compounds present in the wood chemically react with atmospheric oxygen at high temperatures (combustion).
- Entirely new chemical substances are formed: ash, carbon dioxide, carbon monoxide, water vapour, and soot, alongside the liberation of heat and light.
- The ash and smoke cannot be recombined to reconstruct the original wood; hence, it is a permanent and irreversible chemical change.
CBSE Marking Scheme 2026-27
Cutting wood: change in size only, no new substance (physical)1.5 Marks
Burning wood: combustion forming ash/CO₂ irreversibly (chemical)1.5 Marks
Q8
Describe how crystals of copper sulphate are prepared.
3 Marks
✅ CBSE Standard Answer
Pure, large crystals of copper sulphate (CuSO₄·5H₂O) are prepared through the physical purification process of Crystallisation:
Apparatus & Materials: Beaker, tripod stand, wire gauze, bunsen burner, water, dilute sulphuric acid, copper sulphate powder, filter paper, china dish.
Procedure:
- Take approximately 100 mL of water in a clean beaker and add a few drops of dilute sulphuric acid (to prevent hydrolysis).
- Heat the water over a burner on a tripod stand until it starts boiling.
- While boiling, add impure copper sulphate powder gradually while stirring continuously with a glass rod.
- Continue adding copper sulphate powder until no more powder can dissolve; the solution becomes a saturated solution.
- Filter the hot saturated solution using filter paper to remove insoluble solid impurities into a clean china dish.
- Allow the hot filtrate to cool slowly at room temperature undisturbed. Do not shake, stir, or move the solution while cooling.
Observation: After a few hours, beautiful, well-defined, transparent blue triclinic crystals of copper sulphate appear at the bottom of the dish.
CBSE Marking Scheme 2026-27
Heating water with dilute sulphuric acid + preparing saturated solution1.5 Marks
Filtration and slow undisturbed cooling1 Mark
Observation of pure blue crystals (physical change)0.5 Mark
Q9
Explain how painting of an iron gate prevents it from rusting.
2 Marks
✅ CBSE Standard Answer
1. Conditions for Rusting: Rusting of iron is a chemical oxidation reaction that strictly requires the concurrent presence of both oxygen (air) and water / moisture:
4Fe + 3O₂ + 2xH₂O ⎯⎯→ 2Fe₂O₃·xH₂O (Rust)
4Fe + 3O₂ + 2xH₂O ⎯⎯→ 2Fe₂O₃·xH₂O (Rust)
2. Barrier Mechanism of Paint: Applying a coat of paint over the iron gate creates a continuous, impervious physical barrier. This coating completely cuts off direct contact between the underlying metallic iron atoms and atmospheric moisture and oxygen. In the absence of moisture and air, the oxidation reaction cannot initiate, effectively protecting the iron gate from rusting.
CBSE Marking Scheme 2026-27
Essential conditions for rusting (air + moisture)1 Mark
Paint acting as a protective barrier preventing contact with oxygen/water1 Mark
Q10
Explain why rusting of iron objects is faster in coastal areas than in deserts.
2 Marks
✅ CBSE Standard Answer
1. High Humidity in Coastal Areas: Coastal regions (such as Mumbai, Chennai, Kolkata) are surrounded by sea water, creating an atmosphere with very high relative humidity (moisture content). Water vapour readily condenses on iron surfaces, providing the necessary aqueous medium for oxidation.
2. Catalytic Effect of Sea Salts: Coastal air carries tiny droplets of sea spray containing dissolved salts (primarily sodium chloride, NaCl). Saline water acts as an excellent electrolyte, accelerating the electrochemical corrosion process and causing rapid, severe rusting.
3. Arid Conditions in Deserts: In contrast, desert areas (such as Thar) have extremely dry, hot air with virtually negligible atmospheric moisture. Without sufficient water vapour, iron cannot readily react with oxygen, making rusting exceptionally slow.
CBSE Marking Scheme 2026-27
High humidity + presence of salt in coastal air accelerating corrosion1 Mark
Arid, dry conditions in deserts inhibiting rust formation1 Mark
Q11
The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change — A) then it burns (Change — B). The following statements pertain to these changes. Choose the correct one:
(i) Process — A is a chemical change.
(ii) Process — B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
(i) Process — A is a chemical change.
(ii) Process — B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
1 Mark
✅ CBSE Standard Answer
Correct Option: (ii) Process — B is a chemical change.
Scientific Explanation:
- Process A (Liquid LPG → Gaseous LPG): As pressure is released from the cylinder valve, liquefied petroleum gas vaporises into gas. This is strictly a physical change of state; the chemical molecules (butane and propane) remain identical.
- Process B (Combustion of Gas): When ignited over the burner, hydrocarbon gas reacts chemically with atmospheric oxygen to form carbon dioxide, water vapour, heat, and light. This produces entirely new substances and is an irreversible chemical change.
CBSE Marking Scheme 2026-27
Option (ii) with physical vs chemical change distinction1 Mark
Q12
Anaerobic bacteria digest animal waste and produce biogas (Change — A). The biogas is then burnt as fuel (Change — B). The following statements pertain to these changes. Choose the correct one:
(i) Process — A is a chemical change.
(ii) Process — B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
(i) Process — A is a chemical change.
(ii) Process — B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
1 Mark
✅ CBSE Standard Answer
Correct Option: (iii) Both processes A and B are chemical changes.
Scientific Explanation:
- Process A (Biogas Production): Anaerobic methanogenic bacteria biochemically digest and decompose organic cow dung / animal waste into methane and carbon dioxide (biogas). This is a biological chemical change producing new chemical compounds.
- Process B (Combustion of Biogas): The methane in biogas reacts with atmospheric oxygen to produce carbon dioxide, water, and heat energy:
CH₄ + 2O₂ ⎯⎯→ CO₂ + 2H₂O + Heat
Combustion creates entirely new substances and is also a chemical change.
CBSE Marking Scheme 2026-27
Option (iii) Both processes A and B are chemical changes1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Displacement Reaction in Iron Nails & Copper Sulphate
Laboratory Experiment: Priyanka dissolved a spoonful of blue vitriol (copper sulphate) in 100 mL of water and added a few drops of dilute sulphuric acid. She dropped a clean iron nail into the blue solution and left it undisturbed for 45 minutes.
(i) What colour change does the solution undergo, and why?
(ii) What observation is recorded on the surface of the iron nail?
(iii) Write the complete word and chemical equation for this reaction.
(i) What colour change does the solution undergo, and why?
(ii) What observation is recorded on the surface of the iron nail?
(iii) Write the complete word and chemical equation for this reaction.
(i) Colour Shift of Solution: The solution changes from its initial deep blue to a light green colour. This occurs because iron is chemically more reactive than copper; it displaces copper from copper sulphate solution to form iron(II) sulphate (ferrous sulphate, FeSO₄), which is light green.
(ii) Surface of Iron Nail: A distinct reddish-brown powdery coating is deposited on the submerged portion of the iron nail. This reddish-brown coating is pure metallic copper (Cu) displaced from the solution.
(iii) Equation:
Copper sulphate solution (Blue) + Iron (Nail) ⎯⎯→ Iron sulphate solution (Light green) + Copper (Brown deposit)
CuSO₄ (aq) + Fe (s) ⎯⎯→ FeSO₄ (aq) + Cu (s)
⚡ Assertion & Reason (HOTS)
Assertion (A): Ships operating in sea water require frequent replacement of sacrificial zinc blocks attached to their steel hulls.
Reason (R): Zinc is more reactive than iron, so it corrodes preferentially (sacrificial protection), preventing the steel ship hull from rusting in saline ocean water.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Zinc is more reactive than iron, so it corrodes preferentially (sacrificial protection), preventing the steel ship hull from rusting in saline ocean water.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Saline ocean water acts as a strong electrolyte, dramatically accelerating iron corrosion. Large zinc blocks bolted to the submerged steel hull undergo galvanic sacrificial oxidation (zinc oxidises to Zn²⁺ instead of Fe to Fe³⁺). Once the zinc blocks are completely consumed, they must be renewed to safeguard the ship structure.
6
Chapter 6: Respiration in Organisms
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Respiratory Mechanisms
- Cellular Respiration: The intracellular biochemical breakdown of glucose in the presence or absence of oxygen to release usable biological energy (stored as ATP):
- Aerobic Respiration: Occurs in the presence of oxygen; completely oxidises glucose into carbon dioxide, water, and 36–38 ATP molecules:
Glucose + Oxygen ⎯⎯→ Carbon dioxide (6CO₂) + Water (6H₂O) + Energy (38 ATP) - Anaerobic Respiration in Yeast (Fermentation): In the absence of oxygen, glucose is partially broken down into ethyl alcohol, carbon dioxide, and 2 ATP molecules:
Glucose ⎯⎯[In absence of O₂]⎯⎯→ Ethyl alcohol (2C₂H₅OH) + Carbon dioxide (2CO₂) + Energy (2 ATP) - Anaerobic Respiration in Human Skeletal Muscles: Occurs during intense exercise when oxygen supply falls short of muscular demand; produces lactic acid:
Glucose ⎯⎯[In absence of O₂]⎯⎯→ Lactic acid (2C₃H₆O₃) + Energy (2 ATP)
Accumulation of lactic acid causes painful muscle cramps. Relief is obtained through a hot bath or massage, which improves blood circulation and oxygen supply to oxidise lactic acid into CO₂ and H₂O.
- Aerobic Respiration: Occurs in the presence of oxygen; completely oxidises glucose into carbon dioxide, water, and 36–38 ATP molecules:
- Breathing Mechanism in Humans:
- Inhalation (Inspiration): Diaphragm contracts and flattens downwards; ribcage moves upwards and outwards. Chest cavity volume increases, lowering air pressure in lungs. Atmospheric air rushes in through nostrils → pharynx → trachea → bronchi → lungs.
- Exhalation (Expiration): Diaphragm relaxes and curves upwards into dome shape; ribcage moves downwards and inwards. Chest cavity volume decreases, increasing lung air pressure, forcing air out.
- Respiration in Diverse Organisms:
- Insects (Cockroach): Network of air tubes called tracheae opening through small exterior holes called spiracles.
- Earthworm: Breathes through moist, slimy, vascular skin (cutaneous respiration).
- Fish: Breathes dissolved oxygen in water through comb-like, highly vascularised gills.
- Frogs: Dual breathing: through moist skin in water and through a pair of lungs on land.
- Plants: Stomata on leaves and lenticels on bark for gaseous exchange; root hairs absorb oxygen from air spaces between soil particles.
NCERT Intext Questions & Scientific Enquiries
Page 70 — Intext Q1 (Boojho's Query)
Boojho wondered why a person breathes much faster and deeper after running a 100-metre race.
2 Marks
✅ CBSE Standard Answer
Scientific Explanation:
1. High Energy Demand: Vigorous physical running requires immense muscular energy. To meet this demand, muscle cells consume oxygen at a rapid rate to break down glucose into ATP.
2. Oxygen Debt & Lactic Acid: Because oxygen supply cannot keep up with high consumption, muscles temporarily respire anaerobically, producing lactic acid, which causes muscle fatigue.
3. Breathing Rate Acceleration: To repay this 'oxygen debt' and oxidize the accumulated lactic acid into harmless CO₂ and water, the respiratory control center in the brain commands the diaphragm and rib muscles to breathe much faster and deeper.
1. High Energy Demand: Vigorous physical running requires immense muscular energy. To meet this demand, muscle cells consume oxygen at a rapid rate to break down glucose into ATP.
2. Oxygen Debt & Lactic Acid: Because oxygen supply cannot keep up with high consumption, muscles temporarily respire anaerobically, producing lactic acid, which causes muscle fatigue.
3. Breathing Rate Acceleration: To repay this 'oxygen debt' and oxidize the accumulated lactic acid into harmless CO₂ and water, the respiratory control center in the brain commands the diaphragm and rib muscles to breathe much faster and deeper.
CBSE Marking Scheme 2026-27
Increased oxygen demand for cellular respiration and ATP release1 Mark
Repaying oxygen debt to clear accumulated lactic acid in muscles1 Mark
Page 71 — Intext Q2 (Paheli's Query)
Paheli wants to know why we sneeze when we inhale a lot of dust-laden or pepper-filled air.
2 Marks
✅ CBSE Standard Answer
Protective Sneezing Reflex:
1. The nasal cavity is lined with fine hairs (cilia) and sticky mucus designed to trap foreign particulate matter.
2. When we inhale air heavily laden with fine dust, smoke particles, or irritant molecules (like capsaicin in pepper), some particles bypass the nasal hairs and irritate the sensitive mucosal lining of the nasal passage.
3. Sensory nerve receptors trigger a violent, involuntary reflex: the respiratory muscles suddenly contract and forcefully blast out a high-velocity jet of air through the nose and mouth.
4. This explosive expulsion clears the irritating foreign particles from the nasal airway, ensuring that only clean, filtered air reaches the delicate lungs.
1. The nasal cavity is lined with fine hairs (cilia) and sticky mucus designed to trap foreign particulate matter.
2. When we inhale air heavily laden with fine dust, smoke particles, or irritant molecules (like capsaicin in pepper), some particles bypass the nasal hairs and irritate the sensitive mucosal lining of the nasal passage.
3. Sensory nerve receptors trigger a violent, involuntary reflex: the respiratory muscles suddenly contract and forcefully blast out a high-velocity jet of air through the nose and mouth.
4. This explosive expulsion clears the irritating foreign particles from the nasal airway, ensuring that only clean, filtered air reaches the delicate lungs.
CBSE Marking Scheme 2026-27
Irritation of nasal mucosal lining by foreign dust/pepper particles1 Mark
Involuntary explosive expulsion reflex clearing the airway to protect lungs1 Mark
NCERT Textbook Exercise Questions
Q1
Why does an athlete breathe faster and deeper than usual after finishing the race?
2 Marks
✅ CBSE Standard Answer
1. High Energy & Oxygen Debt: During a fast running race, the athlete's leg muscle cells demand immense amounts of energy. Since the oxygen supplied through normal breathing is insufficient, muscle cells respire anaerobically, producing lactic acid.
2. Repaying the Oxygen Debt: After crossing the finish line, the athlete must breathe much faster and deeper to inhale extra oxygen (repaying the "oxygen debt"). This excess oxygen is transported by blood to the muscle cells to completely oxidise and breakdown the accumulated lactic acid into harmless carbon dioxide and water, relieving muscular fatigue.
CBSE Marking Scheme 2026-27
Anaerobic respiration in muscles producing lactic acid during the race1 Mark
Faster breathing providing excess oxygen to break down lactic acid1 Mark
Q2
List the similarities and differences between aerobic and anaerobic respiration.
3 Marks
✅ CBSE Standard Answer
Similarities:
- In both types, glucose serves as the primary cellular fuel substrate to produce usable energy.
- Both processes take place inside living cells through enzyme-catalysed biochemical reactions.
- Both types generate energy stored chemically in ATP molecules.
Differences:
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Presence of Oxygen | Occurs strictly in the presence of oxygen (O₂). | Occurs in the absence of oxygen. |
| Breakdown of Glucose | Complete oxidation of glucose molecules. | Incomplete/partial breakdown of glucose. |
| End Products | Carbon dioxide (CO₂) and Water (H₂O). | Ethyl alcohol + CO₂ (in yeast) OR Lactic acid (in human muscles). |
| Energy Yield | Releases a large amount of energy (~38 ATP per glucose molecule). | Releases very little energy (only 2 ATP per glucose molecule). |
| Occurrence | Most plants, animals, and human cells. | Yeast, certain bacteria, and fatigued human muscle cells. |
CBSE Marking Scheme 2026-27
At least two valid similarities (fuel glucose, ATP generation)1 Mark
Differences in oxygen requirement, end products and energy yield2 Marks
Q3
Why do we often sneeze when we inhale a lot of dust-laden air?
2 Marks
✅ CBSE Standard Answer
1. Nasal Defence Mechanism: The human nasal cavity is lined with fine hairs (cilia) and sticky mucus designed to trap airborne dust particles, pollen, smoke, and microbes.
2. Irritation & Reflex Action: When we inhale air laden with heavy dust, some particles bypass the nasal hairs and directly irritate the delicate, sensitive inner mucosal lining of the nasal cavity.
3. Sneeze Expulsion: This irritation instantly triggers an involuntary protective reflex known as a sneeze. A forceful blast of air is expelled through the nose and mouth, violently dislodging and expelling the foreign irritant particles so that only clean, filtered air enters the delicate lungs.
CBSE Marking Scheme 2026-27
Foreign dust particles irritating the mucosal lining of nasal cavity1 Mark
Sneezing as a protective reflex expelling irritants to safeguard lungs1 Mark
Q4
Take three test-tubes. Fill 3/4th of each with water. Label them A, B and C. Keep a snail in test-tube A, a water plant in test-tube B and in C, keep snail and plant both. Which test-tube would have the highest concentration of CO₂?
3 Marks
✅ CBSE Standard Answer
Test-tube A will have the highest concentration of Carbon Dioxide (CO₂).
Scientific Analysis:
- In Test-tube A (Snail only): The snail performs only respiration; it continuously absorbs dissolved oxygen (O₂) and releases carbon dioxide (CO₂) into the water. Since there is no green plant to utilise the CO₂, carbon dioxide accumulates progressively, reaching the highest concentration.
- In Test-tube B (Water Plant only): The aquatic plant carries out both respiration and photosynthesis. In the presence of light, the rate of photosynthesis is far higher than respiration; the plant rapidly consumes CO₂ from the water to make food, resulting in the lowest CO₂ level.
- In Test-tube C (Snail + Water Plant): The CO₂ exhaled by the snail is actively absorbed and utilised by the water plant for photosynthesis, while the O₂ released by the plant is used by the snail for respiration. A biological equilibrium is maintained, so CO₂ concentration remains moderate.
CBSE Marking Scheme 2026-27
Identifying Test-tube A with highest CO₂ concentration1 Mark
Explanation for Test-tube A (only CO₂ release, no consumer)1 Mark
Comparative explanation for Test-tubes B and C (photosynthesis utilisation)1 Mark
Q5
Tick the correct answer:
(a) In cockroaches, air enters the body through: (i) lungs (ii) gills (iii) spiracles (iv) skin
(b) During heavy exercise, we get cramps in the legs due to the accumulation of: (i) carbon dioxide (ii) lactic acid (iii) alcohol (iv) water
(c) Normal range of breathing rate per minute in an average adult person at rest is: (i) 9–12 (ii) 15–18 (iii) 21–24 (iv) 30–33
(d) During exhalation, the ribs: (i) move outwards (ii) move downwards (iii) move upwards (iv) do not move at all
(a) In cockroaches, air enters the body through: (i) lungs (ii) gills (iii) spiracles (iv) skin
(b) During heavy exercise, we get cramps in the legs due to the accumulation of: (i) carbon dioxide (ii) lactic acid (iii) alcohol (iv) water
(c) Normal range of breathing rate per minute in an average adult person at rest is: (i) 9–12 (ii) 15–18 (iii) 21–24 (iv) 30–33
(d) During exhalation, the ribs: (i) move outwards (ii) move downwards (iii) move upwards (iv) do not move at all
2 Marks
✅ CBSE Standard Answer
(a) (iii) spiracles — Small lateral openings along the sides of the cockroach's body connected to tracheal air tubes.
(b) (ii) lactic acid — Produced by anaerobic breakdown of glucose when muscular oxygen demand outstrips supply.
(c) (ii) 15–18 — An average resting adult human breathes 15 to 18 times per minute (one breath = one inhalation + one exhalation).
(d) (ii) move downwards — During exhalation, the ribs move downwards and inwards while the diaphragm arches upwards into a dome shape.
CBSE Marking Scheme 2026-27
Each correct option chosen (0.5 Mark × 4)2 Marks
Q6
Match the items in Column I with those in Column II:
Column I: (a) Yeast, (b) Diaphragm, (c) Skin, (d) Leaves, (e) Fish, (f) Frog
Column II: (i) Earthworm, (ii) Gills, (iii) Alcohol, (iv) Chest cavity, (v) Stomata, (vi) Lungs and skin, (vii) Tracheae
Column I: (a) Yeast, (b) Diaphragm, (c) Skin, (d) Leaves, (e) Fish, (f) Frog
Column II: (i) Earthworm, (ii) Gills, (iii) Alcohol, (iv) Chest cavity, (v) Stomata, (vi) Lungs and skin, (vii) Tracheae
3 Marks
✅ CBSE Standard Answer
| Column I | Column II | Scientific Rationale |
|---|---|---|
| (a) Yeast | (iii) Alcohol | Anaerobic fermentation by yeast converts sugar into ethanol and CO₂. |
| (b) Diaphragm | (iv) Chest cavity | Muscular sheet forming the flexible floor of the thoracic (chest) cavity. |
| (c) Skin | (i) Earthworm | Cutaneous respiration through thin, vascularised, mucus-moistened skin. |
| (d) Leaves | (v) Stomata | Microscopic pores on leaf epidermis for gaseous exchange. |
| (e) Fish | (ii) Gills | Feathery, blood-rich filaments extracting dissolved oxygen from water. |
| (f) Frog | (vi) Lungs and skin | Amphibian breathing through moist skin in water and lungs on land. |
CBSE Marking Scheme 2026-27
Each correct matching pair (0.5 Mark × 6)3 Marks
Q7
Mark 'T' if the statement is true and 'F' if it is false:
(i) During heavy exercise the breathing rate of a person slows down. (T/F)
(ii) Plants carry out photosynthesis only during the day and respiration only at night. (T/F)
(iii) Frogs breathe through their skins as well as their lungs. (T/F)
(iv) The fishes have lungs for respiration. (T/F)
(v) The size of the chest cavity increases during inhalation. (T/F)
(i) During heavy exercise the breathing rate of a person slows down. (T/F)
(ii) Plants carry out photosynthesis only during the day and respiration only at night. (T/F)
(iii) Frogs breathe through their skins as well as their lungs. (T/F)
(iv) The fishes have lungs for respiration. (T/F)
(v) The size of the chest cavity increases during inhalation. (T/F)
2.5 Marks
✅ CBSE Standard Answer
(i) False (F) — During heavy exercise, breathing rate speeds up drastically (up to 25 breaths/min) to meet higher energy demands.
(ii) False (F) — Respiration is continuous (24 hours, day and night) in all living cells to stay alive; photosynthesis occurs only during the daytime when sunlight is present.
(iii) True (T) — Adult frogs possess lungs on land and breathe through their moist, permeable skin in water.
(iv) False (F) — Fishes breathe through gills, which are specialised to extract dissolved oxygen from water.
(v) True (T) — Outward rib movement and downward diaphragm contraction enlarge the thoracic cavity volume during inhalation.
CBSE Marking Scheme 2026-27
Each correct evaluation with brief rationale (0.5 Mark × 5)2.5 Marks
Q8
Find the respiratory terms for: (i) Air tubes of insects, (ii) Skeletal structures surrounding chest cavity, (iii) Muscular floor of chest cavity, (iv) Tiny pores on the surface of leaf, (v) Small openings on the sides of the body of an insect, (vi) The respiratory organs of human beings.
3 Marks
✅ CBSE Standard Answer
(i) Air tubes of insects: Tracheae
(ii) Skeletal structures surrounding chest cavity: Ribs (Ribcage)
(iii) Muscular floor of chest cavity: Diaphragm
(iv) Tiny pores on the surface of leaf: Stomata
(v) Small openings on the sides of the body of an insect: Spiracles
(vi) The respiratory organs of human beings: Lungs
CBSE Marking Scheme 2026-27
Each correct anatomical term (0.5 Mark × 6)3 Marks
Q9
The mountaineers carry oxygen with them because: (a) At an altitude of more than 5 km there is no air. (b) The amount of air available to a person is less than that available on the ground. (c) The temperature of air is higher than that on the ground. (d) The pressure of air is higher than that on the ground.
1 Mark
✅ CBSE Standard Answer
Correct Option: (b) The amount of air available to a person is less than that available on the ground.
Scientific Explanation: As altitude increases, atmospheric density and atmospheric pressure drop significantly. The air becomes thin (rarefied), meaning there are fewer oxygen molecules per unit volume of air. To prevent hypoxia (oxygen deficiency in tissues and brain), mountaineers must carry portable compressed oxygen cylinders.
CBSE Marking Scheme 2026-27
Option (b) with altitude atmospheric pressure explanation1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Investigating Exhaled Air with Lime Water
Experiment: Rohan set up a clean glass test tube filled with freshly prepared clear lime water [calcium hydroxide, Ca(OH)₂]. Using a clean plastic straw, he gently blew his exhaled breath into the lime water for about 30 seconds.
(i) What change in appearance of the lime water is observed?
(ii) Write the chemical equation explaining this observation.
(iii) What does this test prove regarding the composition of exhaled air compared to inhaled air?
(i) What change in appearance of the lime water is observed?
(ii) Write the chemical equation explaining this observation.
(iii) What does this test prove regarding the composition of exhaled air compared to inhaled air?
(i) Observation: The clear lime water turns cloudy and milky white.
(ii) Chemical Equation: Carbon dioxide in exhaled breath chemically reacts with calcium hydroxide to form an insoluble white precipitate of calcium carbonate (CaCO₃):
Ca(OH)₂ (aq) [Lime water] + CO₂ (g) [In exhaled breath] ⎯⎯→ CaCO₃ (s) [White precipitate] + H₂O (l)
(iii) Scientific Conclusion: Inhaled air contains only 0.04% carbon dioxide, whereas exhaled air contains approximately 4.4% carbon dioxide. This proves conclusively that cellular respiration produces CO₂ as an excretory metabolic byproduct released via the lungs.
⚡ Assertion & Reason (HOTS)
Assertion (A): Taking a hot water bath or getting an oil massage provides quick relief from painful muscle cramps after a strenuous gym workout.
Reason (R): Hot water baths improve systemic blood circulation, which enhances oxygen delivery to muscle cells to completely oxidise accumulated lactic acid into carbon dioxide and water.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Hot water baths improve systemic blood circulation, which enhances oxygen delivery to muscle cells to completely oxidise accumulated lactic acid into carbon dioxide and water.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Muscular cramps stem from the intracellular accumulation of lactic acid under anaerobic conditions. Heat from a warm bath dilates blood vessels (vasodilation), increasing blood flow and flushing muscles with fresh oxygen-rich blood, enabling aerobic enzymes to convert lactic acid back into CO₂ and H₂O.
7
Chapter 7: Transportation in Animals and Plants
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Circulatory / Vascular Systems
- Human Circulatory System:
- Blood Composition: Liquid matrix called plasma containing Red Blood Cells (RBCs) with iron-rich haemoglobin for oxygen transport; White Blood Cells (WBCs) that destroy invading pathogens; and Platelets essential for blood clotting to prevent haemorrhage.
- Blood Vessels: Arteries carry oxygenated blood away from the heart at high pressure (thick, elastic walls, no valves; exception: pulmonary artery); Veins carry deoxygenated blood towards the heart at low pressure (thin walls, possess internal pocket valves preventing backflow; exception: pulmonary vein); Capillaries are microscopic single-cell-thick vessels facilitating nutrient and gas exchange.
- Heart: A hollow, muscular organ roughly the size of a fist, divided into 4 chambers (Right Atrium, Right Ventricle, Left Atrium, Left Ventricle) separated by a muscular septum to completely prevent mixing of oxygen-rich and carbon dioxide-rich blood.
- Human Excretory System: Consists of a pair of bean-shaped Kidneys (filter blood), tube-like Ureters, a muscular Urinary Bladder (stores urine), and a terminal Urethra with an opening. Normal human urine composition: ~95% water, 2.5% urea, 2.5% other toxic metabolic salts. In kidney failure, blood is cleansed periodically using an artificial kidney through dialysis.
- Vascular Transport in Plants:
- Xylem: Continuous tubular network conducting water and dissolved mineral salts unidirectionally from roots upwards to the leaves.
- Phloem: Living vascular tissue translocating food (sugars synthesized by leaves) bidirectionally to all parts of the plant.
- Transpiration: Evaporation of water vapour from leaf stomata. Generates a powerful suction pull (transpirational pull) capable of lifting water columns over 100 metres high in giant trees, while cooling the plant.
NCERT Intext Questions & Scientific Enquiries
Page 81 — Intext Q1 (Boojho's Query)
Boojho wonders why primitive aquatic animals like sponges and Hydra do not possess blood or a circulatory system.
2 Marks
✅ CBSE Standard Answer
Scientific Reason:
1. Direct Water Circulation: Sponges and Hydra are simple multicellular organisms whose body wall consists of only two cell layers in direct contact with their aquatic environment.
2. The natural water in which they reside circulates continuously through their body cavity, bringing in dissolved oxygen and food particles directly to individual cells through simple diffusion.
3. As the water flows out, it carries away carbon dioxide and metabolic wastes directly. Because diffusion distance is microscopic, they require no internal blood or vascular pump.
1. Direct Water Circulation: Sponges and Hydra are simple multicellular organisms whose body wall consists of only two cell layers in direct contact with their aquatic environment.
2. The natural water in which they reside circulates continuously through their body cavity, bringing in dissolved oxygen and food particles directly to individual cells through simple diffusion.
3. As the water flows out, it carries away carbon dioxide and metabolic wastes directly. Because diffusion distance is microscopic, they require no internal blood or vascular pump.
CBSE Marking Scheme 2026-27
Surrounding water entering body bringing food and oxygen directly1 Mark
Outflowing water carrying wastes away, eliminating need for blood1 Mark
Page 88 — Intext Q2 (Boojho's Query)
Boojho wonders why plants absorb such a large quantity of water through their roots if they give off most of it through transpiration.
2 Marks
✅ CBSE Standard Answer
Crucial Roles of Transpiration:
1. Generates Transpiration Pull: Evaporation of water from leaf stomata creates strong suction pressure (transpirational pull) that lifts water and essential dissolved soil minerals hundreds of feet to the crowns of tall trees.
2. Evaporative Cooling: Just as perspiration cools human skin, the continuous evaporation of water dissipates intense solar heat, preventing delicate leaf enzymes from denaturing under midday summer sun.
3. Turgidity & Mechanical Support: Continuous water flow maintains cell turgor, keeping herbaceous leaves upright and firm.
1. Generates Transpiration Pull: Evaporation of water from leaf stomata creates strong suction pressure (transpirational pull) that lifts water and essential dissolved soil minerals hundreds of feet to the crowns of tall trees.
2. Evaporative Cooling: Just as perspiration cools human skin, the continuous evaporation of water dissipates intense solar heat, preventing delicate leaf enzymes from denaturing under midday summer sun.
3. Turgidity & Mechanical Support: Continuous water flow maintains cell turgor, keeping herbaceous leaves upright and firm.
CBSE Marking Scheme 2026-27
Generation of transpirational pull lifting water and minerals1 Mark
Evaporative cooling protecting plant from thermal damage1 Mark
NCERT Textbook Exercise Questions
Q1
Match structures given in Column I with functions given in Column II:
Column I: (i) Stomata, (ii) Xylem, (iii) Root hairs, (iv) Phloem
Column II: (a) Absorption of water, (b) Transpiration, (c) Transport of food, (d) Transport of water, (e) Synthesis of carbohydrates
Column I: (i) Stomata, (ii) Xylem, (iii) Root hairs, (iv) Phloem
Column II: (a) Absorption of water, (b) Transpiration, (c) Transport of food, (d) Transport of water, (e) Synthesis of carbohydrates
2 Marks
✅ CBSE Standard Answer
| Column I (Plant Tissue/Structure) | Column II (Physiological Function) |
|---|---|
| (i) Stomata | (b) Transpiration (and gaseous exchange) |
| (ii) Xylem | (d) Transport of water and dissolved minerals |
| (iii) Root hairs | (a) Absorption of water by increasing root surface area |
| (iv) Phloem | (c) Transport of food (translocation of sugars) |
CBSE Marking Scheme 2026-27
Each correct matching pair (0.5 Mark × 4)2 Marks
Q2
Fill in the blanks:
(i) The blood from the heart is transported to all parts of the body by the _________.
(ii) Haemoglobin is present in _________ cells.
(iii) Arteries and veins are joined by a network of _________.
(iv) The rhythmic expansion and contraction of the heart is called _________.
(v) The main excretory product in human beings is _________.
(vi) Sweat contains water and _________.
(vii) Kidneys eliminate the waste materials in the liquid form called _________.
(viii) Water reaches great heights in the trees because of suction pull caused by _________.
(i) The blood from the heart is transported to all parts of the body by the _________.
(ii) Haemoglobin is present in _________ cells.
(iii) Arteries and veins are joined by a network of _________.
(iv) The rhythmic expansion and contraction of the heart is called _________.
(v) The main excretory product in human beings is _________.
(vi) Sweat contains water and _________.
(vii) Kidneys eliminate the waste materials in the liquid form called _________.
(viii) Water reaches great heights in the trees because of suction pull caused by _________.
4 Marks
✅ CBSE Standard Answer
(i) The blood from the heart is transported to all parts of the body by the arteries.
(ii) Haemoglobin is present in red blood cells (RBCs).
(iii) Arteries and veins are joined by a network of capillaries.
(iv) The rhythmic expansion and contraction of the heart is called heartbeat.
(v) The main excretory product in human beings is urea.
(vi) Sweat contains water and salts.
(vii) Kidneys eliminate the waste materials in the liquid form called urine.
(viii) Water reaches great heights in the trees because of suction pull caused by transpiration.
CBSE Marking Scheme 2026-27
Each correct blank (0.5 Mark × 8)4 Marks
Q3
Choose the correct option:
(a) In plants, water is transported through: (i) xylem (ii) phloem (iii) stomata (iv) root hair
(b) Water absorption through roots can be increased by keeping the plants: (i) in the shade (ii) in dim light (iii) under the fan (iv) covered with a polythene bag
(a) In plants, water is transported through: (i) xylem (ii) phloem (iii) stomata (iv) root hair
(b) Water absorption through roots can be increased by keeping the plants: (i) in the shade (ii) in dim light (iii) under the fan (iv) covered with a polythene bag
2 Marks
✅ CBSE Standard Answer
(a) (i) xylem
Reason: Xylem vessels form a continuous microscopic pipeline from roots through stem into leaf veins to transport water and dissolved minerals upwards.
Reason: Xylem vessels form a continuous microscopic pipeline from roots through stem into leaf veins to transport water and dissolved minerals upwards.
(b) (iii) under the fan
Reason: Air movement from the running fan blows away humid air around the leaves, increasing the rate of evaporation (transpiration). The increased transpiration creates a stronger suction pull, causing the roots to absorb water from the soil much faster.
Reason: Air movement from the running fan blows away humid air around the leaves, increasing the rate of evaporation (transpiration). The increased transpiration creates a stronger suction pull, causing the roots to absorb water from the soil much faster.
CBSE Marking Scheme 2026-27
Part (a): (i) xylem1 Mark
Part (b): (iii) under the fan with transpiration pull explanation1 Mark
Q4
Why is transport of materials necessary in a plant or in an animal? Explain.
3 Marks
✅ CBSE Standard Answer
Transportation of materials is fundamentally essential for the survival and homeostasis of multicellular organisms for the following reasons:
1. Distribution of Nutrients and Water:
- In Plants: Water and minerals absorbed by underground roots must reach leaves for photosynthesis. The glucose prepared by green leaves must be translocated to non-photosynthetic roots, fruits, and growing buds for energy.
- In Animals: Digested nutrients from the intestines must be distributed by blood to trillions of cells across the entire body for cellular respiration and tissue growth.
2. Respiratory Gas Exchange: Oxygen inhaled by the lungs must be carried to all tissues to oxidise food, while carbon dioxide generated during metabolism must be transported back to lungs for expulsion.
3. Elimination of Toxic Waste Products: Cellular metabolism constantly generates toxic nitrogenous byproducts (such as urea and uric acid). An internal circulatory system carries these poisons to excretory organs (kidneys) for filtering and disposal. Without transport, cells would die from toxic accumulation.
CBSE Marking Scheme 2026-27
Transport of water and food nutrients in plants and animals1 Mark
Transport of respiratory gases (O₂ and CO₂)1 Mark
Transport of toxic metabolic wastes to excretory organs1 Mark
Q5
What will happen if there are no platelets in the blood?
2 Marks
✅ CBSE Standard Answer
1. Inability to Clot: Blood platelets (thrombocytes) are specialised cellular fragments responsible for blood coagulation (clotting). When an injury or cut occurs, platelets rupture and release clotting factors that convert soluble fibrinogen into a dark meshwork of insoluble fibrin fibres, forming a dark red clot that seals the wound.
2. Fatal Consequence: If there are no platelets in the blood, bleeding from even a minor scratch or cut will not stop. Continuous, unchecked haemorrhage will cause severe blood loss, drop in blood pressure, organ failure, and eventual death.
CBSE Marking Scheme 2026-27
Platelets responsible for blood coagulation and clot formation1 Mark
Continuous bleeding leading to excessive blood loss and fatal outcome1 Mark
Q6
What are stomata? Give two functions of stomata.
2 Marks
✅ CBSE Standard Answer
Definition: Stomata (singular: stoma) are tiny microscopic pores present predominantly on the epidermal surface of plant leaves. Each pore is flanked and regulated by a pair of kidney-shaped (or bean-shaped) guard cells.
Two Primary Functions:
- Exchange of Gases: They allow the diffusion of carbon dioxide (CO₂) from the atmosphere into the leaf for photosynthesis and the release of oxygen (O₂) into the atmosphere. They also facilitate respiration.
- Transpiration: They allow the evaporation of excess water in the form of water vapour, generating a powerful transpirational suction pull to transport water and cooling the plant.
CBSE Marking Scheme 2026-27
Definition of stomata and guard cells1 Mark
Two functions: gaseous exchange and transpiration1 Mark
Q7
Does transpiration serve any useful function in the plants? Explain.
3 Marks
✅ CBSE Standard Answer
Yes, although plants lose more than 95% of absorbed water through transpiration, it is a "necessary physiological process" serving several indispensable functions:
1. Generation of Transpirational Pull: As water evaporates through leaf stomata, it creates negative hydrostatic pressure (suction force) at the top of xylem columns. Similar to sucking liquid through a straw, this powerful transpirational pull lifts continuous columns of water and dissolved minerals from the roots to the highest tips of tall trees.
2. Mineral Uptake and Distribution: Mineral ions dissolved in soil water (nitrates, phosphates, potassium) are drawn upwards and distributed throughout the plant via transpirational stream.
3. Evaporative Cooling Effect: The evaporation of water absorbs latent heat from leaf tissues, lowering leaf temperatures by 2°C to 5°C. This prevents thermal shock and enzyme denaturing under scorching direct sunlight.
4. Maintaining Cell Turgidity: Transpiration ensures a steady water flow that keeps plant cells turgid, maintaining upright plant posture and leaf shape.
CBSE Marking Scheme 2026-27
Generating suction pull for ascent of sap in xylem1 Mark
Evaporative cooling effect safeguarding delicate leaf enzymes1 Mark
Transport of dissolved mineral salts and turgidity maintenance1 Mark
Q8
What are the components of blood?
3 Marks
✅ CBSE Standard Answer
Blood is a specialised fluid connective tissue composed of four primary components:
1. Plasma: The straw-coloured, fluid liquid matrix constituting about 55% of total blood volume. It consists of 90–92% water with dissolved nutrients (glucose, amino acids), proteins, salts, hormones, and waste materials (carbon dioxide, urea).
2. Red Blood Cells (RBCs / Erythrocytes): Biconcave disc-shaped cells packed with an iron-containing respiratory pigment called haemoglobin. Haemoglobin binds reversibly with oxygen to form oxyhaemoglobin, transporting oxygen from lungs to all body tissues.
3. White Blood Cells (WBCs / Leucocytes): Defence cells that form the body's immune system. They patrol the bloodstream, engulf foreign bacteria, and produce antibodies to neutralise pathogens and fight off infections.
4. Platelets (Thrombocytes): Tiny, disc-shaped cell fragments produced in bone marrow that initiate blood coagulation (clotting) at the site of a cut or injury to prevent fatal blood loss.
CBSE Marking Scheme 2026-27
Plasma: fluid matrix transporting nutrients/wastes0.75 Mark
RBCs: containing haemoglobin for oxygen transport0.75 Mark
WBCs: fighting germs/infections0.75 Mark
Platelets: responsible for blood clotting0.75 Mark
Q9
Why is blood needed by all the parts of a body?
2 Marks
✅ CBSE Standard Answer
Blood is the vital circulating river of life required by every living cell because:
1. Delivery of Oxygen: Every cell requires oxygen continuously to metabolise glucose during cellular respiration; blood carries oxygen from lungs to all peripheral organs.
2. Distribution of Digested Nutrients: It transports glucose, amino acids, fatty acids, vitamins, and minerals absorbed from the small intestine to nourish all body tissues.
3. Waste Removal: It picks up harmful carbon dioxide and nitrogenous wastes (urea) from cells and transports them to the lungs and kidneys for disposal.
4. Protection & Regulation: It carries antibodies and WBCs to fight infections and distributes heat to regulate uniform body temperature (37°C).
CBSE Marking Scheme 2026-27
Supplying oxygen and nutrients to all cells1 Mark
Collecting CO₂/wastes and maintaining body temperature1 Mark
Q10
What makes the blood look red?
1 Mark
✅ CBSE Standard Answer
Blood appears bright red due to the presence of a specialised iron-containing protein pigment called Haemoglobin located inside Red Blood Cells (RBCs). When haemoglobin binds with oxygen in the lungs, it forms bright scarlet-red oxyhaemoglobin.
CBSE Marking Scheme 2026-27
Presence of iron pigment haemoglobin in RBCs1 Mark
Q11
Describe the function of the heart.
3 Marks
✅ CBSE Standard Answer
The human heart acts as a non-stop muscular pump maintaining systemic blood circulation through a double-circulation loop:
1. Four-Chambered Separation: The heart is partitioned into four chambers: two upper receiving atria (Right Atrium, Left Atrium) and two lower pumping ventricles (Right Ventricle, Left Ventricle). A central muscular partition (septum) completely isolates oxygen-rich blood from carbon dioxide-rich blood.
2. Pulmonary Circulation: Deoxygenated blood loaded with CO₂ collected from body tissues enters the Right Atrium, passes into the Right Ventricle, and is pumped via the pulmonary artery to the lungs to discharge CO₂ and absorb fresh oxygen.
3. Systemic Circulation: Freshly oxygenated blood from the lungs returns via pulmonary veins into the Left Atrium, enters the thick-walled Left Ventricle, and is pumped under high pressure through the main artery (Aorta) to all tissues and organs of the entire body.
CBSE Marking Scheme 2026-27
Four chambers and septum preventing mixing of oxygenated/deoxygenated blood1 Mark
Right side pumping deoxygenated blood to lungs for purification1 Mark
Left side pumping oxygenated blood via aorta to all body tissues1 Mark
Q12
Why is it necessary to excrete waste products?
2 Marks
✅ CBSE Standard Answer
1. Continuous Production of Toxins: During normal cellular metabolic processes (such as cellular respiration and protein breakdown), cells generate harmful, toxic metabolic byproducts like urea, uric acid, and carbon dioxide.
2. Toxicity & Cellular Poisoning: If these waste substances are not continuously filtered and eliminated, their concentration builds up in the bloodstream (uraemia). This causes chemical poisoning of vital organs, damages cell membranes, leads to coma, and can be fatal. Hence, excretion is mandatory to maintain physiological homeostasis.
CBSE Marking Scheme 2026-27
Metabolic activities producing toxic nitrogenous wastes like urea1 Mark
Accumulation poisoning cells and disrupting homeostasis1 Mark
Q13
Draw a diagram of the human excretory system and label the various parts.
3 Marks
✅ CBSE Standard Answer
The human excretory system consists of four principal anatomical structures:
1. Pair of Kidneys: Two reddish-brown, bean-shaped organs located against the posterior abdominal wall on either side of the spine. Millions of microscopic nephron filters continuously purify blood, removing urea, excess water, and salts to form urine.
2. Pair of Ureters: Two thin, narrow muscular tubes (~25 cm long) emerging from the hilum of each kidney that conduct urine downwards by peristaltic action.
3. Urinary Bladder: A distensible, hollow muscular reservoir located in the pelvic cavity that temporarily stores urine (holding 300–500 mL) until pressure triggers the urge to urinate.
4. Urethra & Urinary Opening: A short muscular duct leading from the neck of the bladder to an external urinary aperture for micturition (expulsion of urine).
CBSE Marking Scheme 2026-27
Neat anatomical diagrammatic representation with kidneys, ureters, bladder, urethra2 Marks
Correct labelling of all 4 key structures1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Dialysis in Chronic Kidney Disease
Clinical Case: 55-year-old Mohan suffered from chronic uncontrolled hypertension, resulting in complete renal failure (both kidneys stopped functioning). His blood urea level surged dangerously, causing severe nausea, swelling of limbs, and breathlessness. His nephrologist scheduled him for haemodialysis twice a week.
(i) What is the medical term for artificial blood purification when kidneys fail?
(ii) How does a dialysis machine purify blood?
(iii) What permanent medical solution exists if long-term dialysis is no longer viable?
(i) What is the medical term for artificial blood purification when kidneys fail?
(ii) How does a dialysis machine purify blood?
(iii) What permanent medical solution exists if long-term dialysis is no longer viable?
(i) Medical Term: Haemodialysis (Artificial Kidney).
(ii) Mechanism: Blood from an artery in the patient's arm is pumped through a dialysis machine containing thousands of semi-permeable cellophane tubes bathed in a temperature-controlled dialysing fluid (which has the exact same ionic composition as normal blood plasma but zero urea). As patient blood flows through, toxic urea, uric acid, and excess salts diffuse across the semi-permeable walls into the dialysing fluid. The cleansed blood is then warmed, mixed with an anti-coagulant reversal agent, and pumped back into the patient's vein.
(iii) Permanent Solution: Kidney Transplantation — surgically grafting a healthy matching kidney from a compatible living or deceased donor into the patient.
⚡ Assertion & Reason (HOTS)
Assertion (A): Veins have relatively thin walls with internal one-way valves, whereas arteries have thick, elastic walls without valves.
Reason (R): Blood in arteries is pumped directly from the heart under high, pulsating pressure, while blood in veins flows at low pressure and needs valves to prevent backward flow under gravity.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Blood in arteries is pumped directly from the heart under high, pulsating pressure, while blood in veins flows at low pressure and needs valves to prevent backward flow under gravity.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Left ventricular contraction forcefully ejects blood into the aorta at high pressure (~120 mm Hg), requiring thick muscular walls in arteries to prevent bursting. In veins, blood pressure drops to only 5–10 mm Hg; hence, walls are thin, and crescent pocket valves are physiologically essential to ensure blood flows only towards the heart against gravity.
8
Chapter 8: Reproduction in Plants
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Plant Reproduction Modes
- Modes of Plant Reproduction:
- Asexual Reproduction: Multiplication without involving the fusion of gametes or production of seeds. Offspring are genetically identical clones of the single parent. Major types include:
- Vegetative Propagation: New plants arise from vegetative units like roots (sweet potato, dahlia), stems (runners, rhizomes in ginger/turmeric, tubers with "eyes" in potato, stem cuttings in rose), and leaves (notches of leaves in Bryophyllum).
- Budding: Formation of a small daughter bulb-like projection (bud) that detaches and matures (e.g., Yeast).
- Fragmentation: Filamentous organisms break into two or more fragments when mature, each growing into a new individual (e.g., Spirogyra alga).
- Spore Formation: Production of asexual microscopic spores protected by a tough, thick wall to withstand unfavourable conditions (heat, low humidity) which germinate under moisture (e.g., Bread mould / Rhizopus, ferns, mosses).
- Sexual Reproduction: Involves fusion of male and female gametes produced by the reproductive organ of angiosperms — the flower.
- Asexual Reproduction: Multiplication without involving the fusion of gametes or production of seeds. Offspring are genetically identical clones of the single parent. Major types include:
- Anatomy of a Flower:
- Bisexual Flowers: Contain both stamens (male) and pistil (female) — e.g., mustard, rose, petunia.
- Unisexual Flowers: Possess either male stamens (staminate) or female pistil (pistillate) — e.g., corn, papaya, cucumber.
- Stamen (Male organ): Comprises a slender filament and a terminal two-lobed anther containing pollen grains that produce male gametes.
- Pistil / Carpel (Female organ): Comprises the receptive stigma, elongated style, and a basal swollen ovary containing one or more ovules enclosing female gametes (egg cells).
- Pollination & Fertilisation:
- Pollination: Transfer of pollen grains from anther to stigma. Can be Self-pollination (same flower or another flower on same plant) or Cross-pollination (flower of another plant of same species via wind, water, insects).
- Fertilisation: Fusion of male gamete (travelling down a pollen tube) with the female egg cell inside the ovule to produce a diploid zygote. The zygote develops into an embryo; ovule matures into a seed, and the ovary ripens into a fruit.
- Seed Dispersal Mechanisms: Prevents overcrowding, competition for sunlight, water, and minerals. Dispersed by: Wind (winged seeds of drumstick, maple; hairy seeds of aak, dandelion), Water (spongy, fibrous coats in coconut, lotus), Animals (hooked/spiny seeds of Xanthium, Urena), and Explosion/dehiscence (balsam, castor).
NCERT Intext Questions & Scientific Enquiries
Page 95 — Intext Q1 (Boojho's Query)
Boojho wants to know how a piece of potato tuber with an 'eye' develops into a new potato plant.
2 Marks
✅ CBSE Standard Answer
Vegetative Propagation via Potato Tuber:
1. A potato tuber is a swollen underground storage stem. The indentations called 'eyes' are vegetative buds located at stem nodes, protected by scar leaves.
2. When a piece of potato containing an eye is buried in moist, warm soil, the dormant bud absorbs water and nutrients from the starch-rich tuber flesh.
3. The eye sprouts into an aerial shoot growing leaves upwards towards sunlight, while adventitious roots develop downwards into the soil, establishing a vigorous, genetically identical new potato plant.
1. A potato tuber is a swollen underground storage stem. The indentations called 'eyes' are vegetative buds located at stem nodes, protected by scar leaves.
2. When a piece of potato containing an eye is buried in moist, warm soil, the dormant bud absorbs water and nutrients from the starch-rich tuber flesh.
3. The eye sprouts into an aerial shoot growing leaves upwards towards sunlight, while adventitious roots develop downwards into the soil, establishing a vigorous, genetically identical new potato plant.
CBSE Marking Scheme 2026-27
Identification of 'eyes' as vegetative buds on swollen stem tuber1 Mark
Nutrient absorption from tuber producing aerial shoot and adventitious roots1 Mark
Page 99 — Intext Q2 (Paheli's Query)
Paheli asks: Why do flowers of insect-pollinated plants have bright colorful petals, sweet scent, and nectar, whereas wind-pollinated flowers are small, dull, and lack fragrance?
2 Marks
✅ CBSE Standard Answer
Ecological Floral Adaptations:
1. Insect-pollinated flowers: Must actively attract animal pollinators (bees, butterflies). Bright colorful petals act as visual beacons; sweet fragrance guides insects from afar; and sugary nectar serves as a food reward. The pollen is sticky so it adheres to the insect's hairy legs.
2. Wind-pollinated flowers: Wind is an inanimate physical carrier, so energy spent on petals, nectar, and scents would be wasted. Instead, these flowers have exposed hanging stamens, large feathery stigmas to trap drifting pollen, and produce huge quantities of dry, ultra-lightweight pollen that easily floats on gentle breezes.
1. Insect-pollinated flowers: Must actively attract animal pollinators (bees, butterflies). Bright colorful petals act as visual beacons; sweet fragrance guides insects from afar; and sugary nectar serves as a food reward. The pollen is sticky so it adheres to the insect's hairy legs.
2. Wind-pollinated flowers: Wind is an inanimate physical carrier, so energy spent on petals, nectar, and scents would be wasted. Instead, these flowers have exposed hanging stamens, large feathery stigmas to trap drifting pollen, and produce huge quantities of dry, ultra-lightweight pollen that easily floats on gentle breezes.
CBSE Marking Scheme 2026-27
Bright colors, fragrance and nectar as rewards to attract insect vectors1 Mark
Wind-pollinated flowers investing energy in exposed feathery stigmas and light pollen1 Mark
NCERT Textbook Exercise Questions
Q1
Fill in the blanks:
(a) Production of new individuals from the vegetative part of parent is called ______.
(b) A flower may have either male or female reproductive parts. Such a flower is called ______.
(c) The transfer of pollen grains from the anther to the stigma of the same or of another flower of the same kind is known as ______.
(d) The fusion of male and female gametes is termed as ______.
(e) Seed dispersal takes place by means of ______, ______, and ______.
(a) Production of new individuals from the vegetative part of parent is called ______.
(b) A flower may have either male or female reproductive parts. Such a flower is called ______.
(c) The transfer of pollen grains from the anther to the stigma of the same or of another flower of the same kind is known as ______.
(d) The fusion of male and female gametes is termed as ______.
(e) Seed dispersal takes place by means of ______, ______, and ______.
2.5 Marks
✅ CBSE Standard Answer
(a) Production of new individuals from the vegetative part of parent is called vegetative propagation.
(b) A flower may have either male or female reproductive parts. Such a flower is called a unisexual flower.
(c) The transfer of pollen grains from the anther to the stigma of the same or of another flower of the same kind is known as pollination.
(d) The fusion of male and female gametes is termed as fertilisation.
(e) Seed dispersal takes place by means of wind, water, and animals (or explosion of fruits).
CBSE Marking Scheme 2026-27
Each correct blank correctly identified (0.5 Mark × 5)2.5 Marks
Q2
Describe the different methods of asexual reproduction in plants. Give examples.
4 Marks
✅ CBSE Standard Answer
Asexual reproduction is a mode of reproduction in which new individuals are produced from a single parent without the formation and fusion of gametes or seeds. The primary methods are:
1. Vegetative Propagation:
New plants arise from vegetative organs (roots, stems, leaves, buds).
- Stem Tuber: Potato tubers bear "eyes" (buds) covered by scar leaves. Planting an eye piece with a portion of potato gives rise to a new potato plant.
- Stem Cutting: In rose and money plant, a piece of branch cut at a node and planted in soil develops roots and leaves.
- Leaf Buds: In Bryophyllum, margin notches produce plantlets with adventitious roots that detach and establish independently.
- Root Buds: Sweet potato and Dahlia sprout into new plants from their swollen tuberous roots.
2. Budding:
A tiny bulb-like cellular outgrowth called a bud gradually grows on the parent cell, receives a replicated nucleus, and pinches off to form a new daughter cell (which may form chains before detaching).
Example: Unicellular fungus Yeast.
Example: Unicellular fungus Yeast.
3. Fragmentation:
When nutrient and water conditions are favourable, filamentous green algae break into two or more small pieces (fragments). Each fragment rapidly grows through cell division into a complete mature filament.
Example: Spirogyra.
Example: Spirogyra.
4. Spore Formation:
Special asexual reproductive bodies called spores are enclosed inside a hard protective coat (sporangium) to endure extreme drying and heat. When conditions become warm and humid, the sporangium bursts, liberating microscopic spores that germinate into new mycelia.
Example: Bread mould (Rhizopus), mosses, and ferns.
Example: Bread mould (Rhizopus), mosses, and ferns.
CBSE Marking Scheme 2026-27
Vegetative propagation definition with suitable examples1 Mark
Budding mechanism with Yeast example1 Mark
Fragmentation mechanism with Spirogyra example1 Mark
Spore formation explanation with Bread mould example1 Mark
Q3
Explain what you understand by sexual reproduction.
2 Marks
✅ CBSE Standard Answer
Definition: Sexual reproduction is the biological process of creating new offspring by the union of two specialized sex cells (gametes) — a male gamete and a female gamete — typically originating from two parents (or from both male and female organs of a bisexual organism).
Key Features in Flowering Plants:
- The flower is the reproductive organ of angiosperms.
- The stamen (male reproductive organ) produces pollen grains containing male gametes.
- The pistil / carpel (female reproductive organ) bears ovules containing female gametes (egg cells).
- Pollination transfers pollen to the stigma, leading to fertilisation (syngamy) to form a single diploid cell called a zygote.
- The zygote develops into an embryo contained within a protective seed, which germinates under favourable moisture and warmth to yield a genetically unique plant.
CBSE Marking Scheme 2026-27
Clear definition highlighting fusion of male and female gametes1 Mark
Role of flower parts, zygote formation, and seed development1 Mark
Q4
State the main difference between asexual and sexual reproduction.
3 Marks
✅ CBSE Standard Answer
| Feature / Parameter | Asexual Reproduction | Sexual Reproduction |
|---|---|---|
| Number of Parents | Only one single parent is involved. | Usually two parents (male and female) are involved. |
| Gamete Formation & Fertilisation | No gametes are produced; no fertilisation takes place. | Male and female gametes are produced and undergo fertilisation. |
| Involvement of Seeds | New plants develop without seeds (from stems, roots, leaves, buds, or spores). | Seeds are invariably formed within fruits and germinate into new plants. |
| Genetic Variation | Offspring are exact morphological and genetic clones of the parent. | Offspring inherit characters from both parents, creating genetic variation and diversity. |
| Speed & Rate of Multiplication | Rapid multiplication process; produces a large population quickly. | Comparatively slower process requiring flowering, pollination, and seed maturation. |
| Examples | Potato (tubers), Yeast (budding), Spirogyra (fragmentation). | Mustard, Rose, Pea, Mango, Wheat, Papaya. |
CBSE Marking Scheme 2026-27
Any 3 well-distinguished comparison points (1 Mark each)3 Marks
Q5
Sketch the reproductive parts of a flower.
3 Marks
✅ CBSE Standard Answer
The flower contains two essential reproductive whorls: the Stamen (androecium) and the Pistil / Carpel (gynoecium).
1. Stamen (Male Reproductive Organ):
- Anther: The terminal, bilobed sac that produces and stores microscopic pollen grains (which house the male gametes).
- Filament: The slender, elongated stalk that supports and positions the anther for optimal pollen dispersal.
2. Pistil / Carpel (Female Reproductive Organ):
- Stigma: The sticky or feathery top surface designed to capture and anchor landed pollen grains.
- Style: The narrow, elongated neck connecting the stigma to the ovary, through which the pollen tube grows.
- Ovary: The basal, swollen chamber containing one or more egg-containing ovules. Upon fertilisation, the ovary ripens into the fruit.
- Ovule: The female reproductive structure inside the ovary containing the female gamete (egg cell), which transforms into the seed.
Diagrammatic Layout for Board Exams:
Draw two distinct sketches:
Draw two distinct sketches:
- Stamen: A slender stalk labelled Filament leading to a two-lobed top labelled Anther.
- Pistil: A vase-shaped structure with a top flat disc labelled Stigma, a central tube labelled Style, and a swollen lower base labelled Ovary with rounded inner bodies labelled Ovules.
CBSE Marking Scheme 2026-27
Neat diagram of Stamen with Anther and Filament labelled1.5 Marks
Neat diagram of Pistil with Stigma, Style, Ovary, and Ovule labelled1.5 Marks
Q6
Explain the difference between self-pollination and cross-pollination.
3 Marks
✅ CBSE Standard Answer
| Feature | Self-Pollination (Autogamy / Geitonogamy) | Cross-Pollination (Allogamy / Xenogamy) |
|---|---|---|
| Definition | Transfer of pollen grains from the anther to the stigma of the same flower or another flower on the same plant. | Transfer of pollen grains from the anther of one flower to the stigma of a flower on another plant of the same species. |
| External Agents | External pollinating agents (insects, wind, water) are usually not required. | Requires external pollinating agents such as bees, butterflies, birds, wind, or water. |
| Floral Adaptations | Flowers need not be large, showy, scent-producing, or rich in nectar. | Flowers are usually large, brightly colored, possess fragrant scent and sweet nectar to attract pollinators (or produce dry, powdery pollen for wind). |
| Genetic Variation | Does not introduce genetic variations; preserves pure parental traits. | Combines genes from two parent plants, resulting in high genetic diversity and stronger adaptability. |
| Examples | Pea, peanut, wheat, rice. | Mustard, apple, sunflower, corn, papaya. |
CBSE Marking Scheme 2026-27
Accurate definition of self-pollination and cross-pollination1 Mark
Differences based on pollinating agents and genetic variations1 Mark
Appropriate plant examples for both1 Mark
Q7
How does the process of fertilisation take place in flowers?
3 Marks
✅ CBSE Standard Answer
Fertilisation is the process in which the male gamete fuses with the female gamete to form a single diploid cell called a zygote. In flowering plants, it occurs in the following sequential stages:
Step 1: Pollen Germination:
After landing on the compatible receptive surface of the stigma, the pollen grain absorbs moisture and sugary nutrient secretions, germinating to produce a narrow, thin pollen tube.
Step 2: Growth down the Style:
The pollen tube grows downwards through the tissues of the style towards the ovary, carrying two male gametes in its tip.
Step 3: Entry into the Ovule:
The pollen tube enters the ovary and penetrates an ovule through a microscopic aperture called the micropyle.
Step 4: Syngamy / Fusion:
The tip of the pollen tube bursts open, releasing the male gamete, which fuses directly with the female egg cell situated within the ovule. This union results in the formation of a zygote.
Post-Fertilisation Changes:
- The zygote divides repeatedly to develop into an embryo.
- The ovule develops a tough seed coat and matures into a seed.
- The ovary enlarges, ripens, and transforms into a succulent or dry fruit.
- Petals, sepals, and stamens wither and fall off.
CBSE Marking Scheme 2026-27
Pollen germination and pollen tube elongation description1 Mark
Fusion of male gamete with egg cell in ovule to form zygote1 Mark
Post-fertilisation development into seed and fruit1 Mark
Q8
Describe the various ways by which seeds are dispersed.
4 Marks
✅ CBSE Standard Answer
Seed dispersal is the natural scattering or transport of seeds away from parent plants. It is ecologically vital because it avoids overcrowding, minimizes intense competition for water, sunlight, and soil minerals, and enables species to invade new geographical habitats. The four major mechanisms are:
1. Dispersal by Wind:
Seeds are lightweight, small, and equipped with special aerodynamic structures:
- Winged seeds: Drumstick (Moringa) and Maple seeds have thin papery wings that catch wind currents and glide over long distances.
- Hairy seeds and tufts: Aak (Calotropis) and Dandelion have parachutes or tufts of fine silk-like hairs; sunflower fruits possess a hairy pappus that floats on breezes.
- Microscopic spores: Extremely light spores of ferns and mosses blow away effortlessly.
2. Dispersal by Water:
Seeds or fruits develop light, fibrous, or spongy outer coats that trap air, enabling them to float on surface water streams and ocean currents.
Examples: Coconut has a buoyant fibrous mesocarp; Lotus has a spongy fruit that floats down rivers.
Examples: Coconut has a buoyant fibrous mesocarp; Lotus has a spongy fruit that floats down rivers.
3. Dispersal by Animals:
Seeds possess physical clinging adaptations or sweet fleshy pulp:
- Hooked / Spiny seeds: Xanthium and Urena have hooked spines that latch tenaciously onto the fur of grazing animals or human clothing, falling off miles away.
- Edible fruits: Birds and mammals consume juicy fruits (figs, guava, berries); the indigestible hard seeds pass unharmed through the digestive tract and are deposited far away in nutrient-rich droppings.
4. Dispersal by Explosive Mechanism (Dehiscence):
Upon ripening and drying under solar heat, extreme mechanical tension builds up inside the fruit wall, causing the pericarp to suddenly burst open with explosive force, catapulting seeds several metres away from the parent plant.
Examples: Balsam, Castor, and Pea pods.
Examples: Balsam, Castor, and Pea pods.
CBSE Marking Scheme 2026-27
Wind dispersal with winged/hairy examples (Drumstick, Maple, Aak)1 Mark
Water dispersal with buoyant structures (Coconut, Lotus)1 Mark
Animal dispersal with hooked seeds (Xanthium, Urena)1 Mark
Explosive dehiscence mechanism (Balsam, Castor)1 Mark
Q9
Match items in Column I with those in Column II:
Column I: (a) Bud, (b) Eyes, (c) Fragmentation, (d) Wings, (e) Spores
Column II: (i) Maple, (ii) Spirogyra, (iii) Yeast, (iv) Bread mould, (v) Potato, (vi) Rose
Column I: (a) Bud, (b) Eyes, (c) Fragmentation, (d) Wings, (e) Spores
Column II: (i) Maple, (ii) Spirogyra, (iii) Yeast, (iv) Bread mould, (v) Potato, (vi) Rose
2.5 Marks
✅ CBSE Standard Answer
| Column I (Structure / Feature) | Column II (Matched Organism) |
|---|---|
| (a) Bud | (iii) Yeast (reproduces asexually by budding) |
| (b) Eyes | (v) Potato (vegetative buds on tubers) |
| (c) Fragmentation | (ii) Spirogyra (green alga breaking into filaments) |
| (d) Wings | (i) Maple (winged seeds for aerodynamic wind dispersal) |
| (e) Spores | (iv) Bread mould (asexual spores in sporangia) |
Note: (vi) Rose reproduces via stem cuttings (vegetative propagation), so it remains an extra unmatched distractor.
CBSE Marking Scheme 2026-27
Each correctly matched pair (0.5 Mark × 5)2.5 Marks
Q10
Tick the correct answer:
(a) The reproductive part of a plant is the: (i) leaf, (ii) stem, (iii) root, (iv) flower.
(b) The process of fusion of the male and the female gametes is called: (i) fertilisation, (ii) pollination, (iii) reproduction, (iv) seed formation.
(c) Mature ovary forms the: (i) seed, (ii) stamen, (iii) pistil, (iv) fruit.
(d) A spore-producing plant is: (i) rose, (ii) bread mould, (iii) potato, (iv) ginger.
(e) Bryophyllum can reproduce by its: (i) stem, (ii) leaves, (iii) roots, (iv) flower.
(a) The reproductive part of a plant is the: (i) leaf, (ii) stem, (iii) root, (iv) flower.
(b) The process of fusion of the male and the female gametes is called: (i) fertilisation, (ii) pollination, (iii) reproduction, (iv) seed formation.
(c) Mature ovary forms the: (i) seed, (ii) stamen, (iii) pistil, (iv) fruit.
(d) A spore-producing plant is: (i) rose, (ii) bread mould, (iii) potato, (iv) ginger.
(e) Bryophyllum can reproduce by its: (i) stem, (ii) leaves, (iii) roots, (iv) flower.
2.5 Marks
✅ CBSE Standard Answer
(a) (iv) flower
Reason: While leaves, stems, and roots are vegetative parts, the flower contains the specialized male (stamen) and female (pistil) reproductive structures.
Reason: While leaves, stems, and roots are vegetative parts, the flower contains the specialized male (stamen) and female (pistil) reproductive structures.
(b) (i) fertilisation
Reason: Fertilisation (syngamy) is the precise biological union of male and female gametic nuclei to produce a zygote.
Reason: Fertilisation (syngamy) is the precise biological union of male and female gametic nuclei to produce a zygote.
(c) (iv) fruit
Reason: Following successful fertilisation, the ovules transform into seeds while the surrounding ovary wall swells and ripens into the fruit.
Reason: Following successful fertilisation, the ovules transform into seeds while the surrounding ovary wall swells and ripens into the fruit.
(d) (ii) bread mould
Reason: Bread mould (Rhizopus) forms globular sporangia bearing asexual resistant spores.
Reason: Bread mould (Rhizopus) forms globular sporangia bearing asexual resistant spores.
(e) (ii) leaves
Reason: Bryophyllum features adventitious buds along the serrated notches of its leaf margins, which germinate into plantlets.
Reason: Bryophyllum features adventitious buds along the serrated notches of its leaf margins, which germinate into plantlets.
CBSE Marking Scheme 2026-27
Each correct option with valid scientific reason (0.5 Mark × 5)2.5 Marks
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Commercial Horticulture & Vegetative Propagation
Horticultural Application: An agricultural scientist advises a farmer growing seedless Thompson grapes and high-yielding Alphonso mangoes to exclusively practice vegetative propagation (cuttings and grafting) rather than growing plants from seeds.
(i) Why cannot seedless grapes be propagated through sexual reproduction?
(ii) State two major commercial advantages vegetative propagation offers over growing from seeds.
(iii) What is the fundamental disadvantage of monoculture clones produced through vegetative propagation?
(i) Why cannot seedless grapes be propagated through sexual reproduction?
(ii) State two major commercial advantages vegetative propagation offers over growing from seeds.
(iii) What is the fundamental disadvantage of monoculture clones produced through vegetative propagation?
(i) Reason for Seedless Grapes: Seedless grape varieties do not produce viable seeds containing embryos; hence sexual reproduction through seed germination is biologically impossible. They can only be multiplied through stem cuttings or grafting.
(ii) Commercial Advantages:
1. Speed and Earlier Maturity: Vegetatively propagated plants bear flowers and fruits much earlier than seedlings grown from seeds.
2. Exact Genetic Quality: Offspring are exact clones, ensuring 100% preservation of desirable traits (taste, fruit size, disease resistance, sweetness) without genetic variation.
(iii) Biological Disadvantage: Because all plants in the orchard share identical genetic makeup, there is zero genetic diversity. If a new disease or pest strikes, every single plant will be equally susceptible, risking total crop wipeout.
⚡ Assertion & Reason (HOTS)
Assertion (A): Nature has evolved intricate mechanisms to favor cross-pollination over self-pollination in most flowering plants.
Reason (R): Cross-pollination brings about new combinations of genes from two distinct parent plants, creating healthier offspring with higher adaptability to changing environments.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Cross-pollination brings about new combinations of genes from two distinct parent plants, creating healthier offspring with higher adaptability to changing environments.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Continuous self-pollination leads to inbreeding depression, accumulation of harmful recessive alleles, and reduced vigour. Cross-pollination introduces novel genetic variations, enhances environmental fitness, and produces vigorous, disease-resistant seeds.
9
Chapter 9: Motion and Time
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Kinematic Equations
- Motion & Speed:
- Speed: The total distance covered by an object divided by the total time taken to cover that distance.
Speed = Total Distance Covered / Total Time Taken - SI Units: Basic unit of distance is metre (m); basic unit of time is second (s). Hence, the SI basic unit of speed is metre per second (m/s). Larger commercial unit is kilometre per hour (km/h).
Conversion:1 km/h = (1000 m) / (3600 s) = 5/18 m/s - Uniform vs Non-uniform Motion: An object moving along a straight line with a constant speed is in uniform motion. If the speed keeps changing with time, the motion is non-uniform.
- Speed: The total distance covered by an object divided by the total time taken to cover that distance.
- Measurement of Time & Periodic Motion:
- Clocks and watches rely on regular, repeating periodic events.
- Simple Pendulum: Consists of a small metallic sphere (bob) suspended from a rigid stand by a light thread.
- Oscillation: One complete to-and-fro motion of the bob from one extreme position to another and back.
- Time Period (T): The time taken by the pendulum bob to complete one full oscillation.
Time Period (T) = Total Time Taken / Number of OscillationsDiscovery: Galileo Galilei observed that the time period of a simple pendulum depends only on the length of the suspension string, and remains constant regardless of the amplitude of swing.
- Distance-Time Graphs:
- A graphical representation plotting time on the horizontal X-axis and distance on the vertical Y-axis.
- A straight line sloping upwards indicates uniform speed (constant speed).
- A horizontal line parallel to the time axis indicates an object at rest (speed = 0).
- A curved line indicates non-uniform speed (accelerating or decelerating motion).
- The steeper the slope (gradient) of the distance-time graph, the greater is the speed of the object.
NCERT Intext Questions & Scientific Enquiries
Page 108 — Intext Q1 (Boojho's Query)
Boojho was wondering how people in ancient times measured time without pendulum clocks or modern electronic watches.
2 Marks
✅ CBSE Standard Answer
Ancient Periodic Timekeepers:
Ancient civilizations tracked time using regular, repeating natural periodic events and ingenious devices:
Ancient civilizations tracked time using regular, repeating natural periodic events and ingenious devices:
- Solar Day & Sundials: Observed sunrise to sunset; shadow lengths cast by gnomons on calibrated stone dials (e.g. Jantar Mantar sundials).
- Water Clocks (Clepsydra): Regulated outflow of water through a small calibrated orifice from one vessel into another.
- Sand Hourglasses: Flow of fine sand through a narrow glass waist under gravity over a fixed time duration.
- Lunar Phases: New moon to full moon tracked months; seasonal solar cycles tracked calendar years.
CBSE Marking Scheme 2026-27
Any 3 ancient methods explained (Sundials, water clocks, sand glasses, lunar cycles)2 Marks
Page 111 — Intext Q2 (Paheli's Query)
Paheli asks: What is the difference between a speedometer and an odometer installed on a vehicle's dashboard?
2 Marks
✅ CBSE Standard Answer
| Parameter | Speedometer | Odometer |
|---|---|---|
| Measurement | Measures instantaneous speed of the vehicle at that precise moment. | Measures total distance traversed by the vehicle throughout its lifespan. |
| Units | Calibrated directly in km/h (kilometres per hour). | Calibrated in km (kilometres, with tenths of km). |
CBSE Marking Scheme 2026-27
Speedometer measuring instantaneous speed in km/h1 Mark
Odometer measuring cumulative distance in km1 Mark
NCERT Textbook Exercise Questions
Q1
Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running.
(ii) Motion of a horse pulling a cart on a straight road.
(iii) Motion of a child in a merry-go-round.
(iv) Motion of a child on a see-saw.
(v) Motion of the hammer of an electric bell.
(vi) Motion of a train on a straight bridge.
(i) Motion of your hands while running.
(ii) Motion of a horse pulling a cart on a straight road.
(iii) Motion of a child in a merry-go-round.
(iv) Motion of a child on a see-saw.
(v) Motion of the hammer of an electric bell.
(vi) Motion of a train on a straight bridge.
3 Marks
✅ CBSE Standard Answer
| S.No. | Example of Motion | Classification / Type of Motion | Scientific Reason |
|---|---|---|---|
| (i) | Motion of hands while running | Oscillatory motion | Hands execute a repetitive to-and-fro movement about a mean position. |
| (ii) | Horse pulling a cart on a straight road | Motion along a straight line (Rectilinear motion) | The movement occurs along a unidirectional straight trajectory. |
| (iii) | Child in a merry-go-round | Circular motion | The child moves along a circular path at a constant radial distance from the central axis. |
| (iv) | Child on a see-saw | Oscillatory motion | The see-saw moves repeatedly up and down about a central fulcrum pivot. |
| (v) | Hammer of an electric bell | Oscillatory motion | The metallic hammer strikes back and forth repeatedly against the gong. |
| (vi) | Train on a straight bridge | Motion along a straight line (Rectilinear motion) | The train tracks form a straight linear path across the bridge. |
CBSE Marking Scheme 2026-27
Each correct classification with justification (0.5 Mark × 6)3 Marks
Q2
Which of the following are not correct?
(i) The basic unit of time is second.
(ii) Every object moves with a constant speed.
(iii) Distances between two cities are measured in kilometres.
(iv) The time period of a given pendulum is not constant.
(v) The speed of a train is expressed in m/h.
(i) The basic unit of time is second.
(ii) Every object moves with a constant speed.
(iii) Distances between two cities are measured in kilometres.
(iv) The time period of a given pendulum is not constant.
(v) The speed of a train is expressed in m/h.
2.5 Marks
✅ CBSE Standard Answer
The statements that are not correct (incorrect) are:
(ii) Incorrect: "Every object moves with a constant speed."
Correction: Most natural objects move with variable (non-uniform) speeds because of traffic, friction, turns, starting, and stopping.
Correction: Most natural objects move with variable (non-uniform) speeds because of traffic, friction, turns, starting, and stopping.
(iv) Incorrect: "The time period of a given pendulum is not constant."
Correction: The time period of a given pendulum of fixed length is constant (isochronism of pendulums discovered by Galileo).
Correction: The time period of a given pendulum of fixed length is constant (isochronism of pendulums discovered by Galileo).
(v) Incorrect: "The speed of a train is expressed in m/h."
Correction: Because trains travel long distances at high speeds, their speed is practically expressed in km/h (kilometres per hour) or in SI units as m/s.
Correction: Because trains travel long distances at high speeds, their speed is practically expressed in km/h (kilometres per hour) or in SI units as m/s.
Statements (i) and (iii) are completely correct: The SI basic unit of time is the second (s), and geographical distances between cities are measured in kilometres (km).
CBSE Marking Scheme 2026-27
Identification of statements (ii), (iv), and (v) as incorrect with corrections2.5 Marks
Q3
A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
2 Marks
✅ CBSE Standard Answer
Given Data:
Total number of complete oscillations (n) = 20
Total time taken (t) = 32 seconds
Total number of complete oscillations (n) = 20
Total time taken (t) = 32 seconds
Formula:
The time period (T) of a simple pendulum is defined as the time required to complete one single oscillation:
The time period (T) of a simple pendulum is defined as the time required to complete one single oscillation:
Time Period (T) = Total Time Taken / Total Number of Oscillations
Calculation:
T = 32 / 20 = 1.6 seconds
Conclusion:
The time period of the simple pendulum is 1.6 seconds.
The time period of the simple pendulum is 1.6 seconds.
CBSE Marking Scheme 2026-27
Statement of definition and formula for time period1 Mark
Accurate substitution and final answer with unit (1.6 s)1 Mark
Q4
The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
2 Marks
✅ CBSE Standard Answer
Given Data:
Distance covered by the train (D) = 240 km
Time taken to travel the distance (t) = 4 hours (h)
Distance covered by the train (D) = 240 km
Time taken to travel the distance (t) = 4 hours (h)
Formula:
Speed = Distance / Time
Calculation:
Speed = 240 km / 4 h = 60 km/h
In SI units (m/s):Speed = 60 × (5/18) = 300 / 18 ≈ 16.67 m/s
Conclusion:
The speed of the train is 60 km/h (or 16.67 m/s).
The speed of the train is 60 km/h (or 16.67 m/s).
CBSE Marking Scheme 2026-27
Formula Speed = Distance / Time0.5 Mark
Step-by-step calculation with correct units (60 km/h)1.5 Marks
Q5
The odometer of a car reads 57321.0 km when the clock shows the time 08:30 AM. What is the distance moved by the car, if at 08:50 AM, the odometer reading has changed to 57336.0 km? Calculate the speed of the car in km/min during this time. Express the speed in km/h also.
3 Marks
✅ CBSE Standard Answer
Step 1: Calculate Distance Covered:
Initial odometer reading at 08:30 AM = 57321.0 km
Final odometer reading at 08:50 AM = 57336.0 km
Initial odometer reading at 08:30 AM = 57321.0 km
Final odometer reading at 08:50 AM = 57336.0 km
Distance = Final Reading - Initial Reading = 57336.0 - 57321.0 = 15.0 km
Step 2: Calculate Elapsed Time:
Time elapsed = 08:50 AM - 08:30 AM = 20 minutes
Time elapsed = 08:50 AM - 08:30 AM = 20 minutes
Step 3: Calculate Speed in km/min:
Speed = Distance / Time = 15 km / 20 min = 0.75 km/min
Step 4: Express Speed in km/h:
Since 1 hour = 60 minutes, time in hours = 20/60 h = 1/3 h.
Since 1 hour = 60 minutes, time in hours = 20/60 h = 1/3 h.
Speed = 15 km / (1/3 h) = 15 × 3 = 45 km/h
(Alternatively: Speed in km/h = 0.75 km/min × 60 min/h = 45 km/h)
Final Summary:
• Distance moved = 15.0 km
• Speed in km/min = 0.75 km/min
• Speed in km/h = 45 km/h
• Distance moved = 15.0 km
• Speed in km/min = 0.75 km/min
• Speed in km/h = 45 km/h
CBSE Marking Scheme 2026-27
Distance calculation (15 km) and time interval (20 min)1 Mark
Speed in km/min (0.75 km/min)1 Mark
Conversion and speed in km/h (45 km/h)1 Mark
Q6
Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/s, calculate the distance between her house and the school.
2 Marks
✅ CBSE Standard Answer
Given Data:
Speed of bicycle (v) = 2 m/s
Time taken (t) = 15 minutes
Speed of bicycle (v) = 2 m/s
Time taken (t) = 15 minutes
Unit Conversion:
To use the speed in m/s, time must be converted from minutes into seconds:
To use the speed in m/s, time must be converted from minutes into seconds:
Time (t) = 15 × 60 s = 900 seconds
Formula:
Distance = Speed × Time
Calculation:
Distance = 2 m/s × 900 s = 1800 metres
Converting to kilometres:Distance = 1800 / 1000 = 1.8 km
Conclusion:
The distance between Salma's house and her school is 1800 m (or 1.8 km).
The distance between Salma's house and her school is 1800 m (or 1.8 km).
CBSE Marking Scheme 2026-27
Conversion of time to seconds (900 s)0.5 Mark
Formula and calculation yielding 1800 m or 1.8 km1.5 Marks
Q7
Show the shape of the distance-time graph for the motion in the following cases:
(i) A car moving with a constant speed.
(ii) A car parked on a side road.
(i) A car moving with a constant speed.
(ii) A car parked on a side road.
3 Marks
✅ CBSE Standard Answer
Case (i): A car moving with a constant speed:
• Shape of Graph: A straight line inclined at an angle to the time axis (sloping upwards linearly from the origin).
• Scientific Explanation: When an object covers equal distances in equal intervals of time, the ratio Distance / Time remains constant. Hence, the graph is a linear upward diagonal line representing uniform motion.
• Shape of Graph: A straight line inclined at an angle to the time axis (sloping upwards linearly from the origin).
• Scientific Explanation: When an object covers equal distances in equal intervals of time, the ratio Distance / Time remains constant. Hence, the graph is a linear upward diagonal line representing uniform motion.
Case (ii): A car parked on a side road:
• Shape of Graph: A horizontal straight line parallel to the time axis (X-axis) at a fixed distance value.
• Scientific Explanation: As time progresses continuously, the car remains stationary at the exact same location, so distance does not change (Slope = 0, Speed = 0).
• Shape of Graph: A horizontal straight line parallel to the time axis (X-axis) at a fixed distance value.
• Scientific Explanation: As time progresses continuously, the car remains stationary at the exact same location, so distance does not change (Slope = 0, Speed = 0).
Exam Diagram Note:
Always label Time on the horizontal X-axis (with units, e.g., seconds or minutes) and Distance on the vertical Y-axis (with units, e.g., metres or kilometres).
Always label Time on the horizontal X-axis (with units, e.g., seconds or minutes) and Distance on the vertical Y-axis (with units, e.g., metres or kilometres).
CBSE Marking Scheme 2026-27
Shape and explanation for constant speed (straight inclined line)1.5 Marks
Shape and explanation for parked car (horizontal line parallel to X-axis)1.5 Marks
Q8
Which of the following relations is correct?
(i) Speed = Distance × Time
(ii) Speed = Distance / Time
(iii) Speed = Time / Distance
(iv) Speed = 1 / (Distance × Time)
(i) Speed = Distance × Time
(ii) Speed = Distance / Time
(iii) Speed = Time / Distance
(iv) Speed = 1 / (Distance × Time)
1 Mark
✅ CBSE Standard Answer
Correct Option: (ii) Speed = Distance / Time
Scientific Definition: Speed is defined as the rate of change of distance with respect to time, or the distance covered by an object per unit time.
CBSE Marking Scheme 2026-27
Correct option selection with definition1 Mark
Q9
The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s
1 Mark
✅ CBSE Standard Answer
Correct Option: (iv) m/s (metre per second)
Scientific Reason: In the International System of Units (SI), the base unit of length is the metre (m) and the base unit of time is the second (s). Since Speed = Distance / Time, the basic derived unit of speed is metres divided by seconds, written as m/s.
CBSE Marking Scheme 2026-27
Selection of m/s with SI derivation1 Mark
Q10
A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by the car is:
(i) 100 km
(ii) 25 km
(iii) 15 km
(iv) 10 km
(i) 100 km
(ii) 25 km
(iii) 15 km
(iv) 10 km
2 Marks
✅ CBSE Standard Answer
Correct Option: (ii) 25 km
Detailed Step-by-Step Calculation:
• Time duration for each part = 15 minutes = 15 / 60 hours = 1/4 hour = 0.25 h.
• Part 1: Distance covered in first 15 minutes (d₁):
• Part 2: Distance covered in next 15 minutes (d₂):
• Total Distance:
• Time duration for each part = 15 minutes = 15 / 60 hours = 1/4 hour = 0.25 h.
• Part 1: Distance covered in first 15 minutes (d₁):
d₁ = Speed₁ × Time₁ = 40 km/h × (1/4) h = 10 km
• Part 2: Distance covered in next 15 minutes (d₂):
d₂ = Speed₂ × Time₂ = 60 km/h × (1/4) h = 15 km
• Total Distance:
Total Distance = d₁ + d₂ = 10 km + 15 km = 25 km
CBSE Marking Scheme 2026-27
Calculation of individual distances (10 km and 15 km)1 Mark
Total sum (25 km) matching option (ii)1 Mark
Q11
Suppose the two photographs, shown in Fig. 9.1 and Fig. 9.2, had been taken at an interval of 10 seconds. If a distance of 100 metres is shown by 1 cm in these photographs, calculate the speed of the fastest blue car (assuming it covers 2 cm on the photograph scale).
3 Marks
✅ CBSE Standard Answer
Given Data:
Time interval between the two photographs (t) = 10 s
Scale of photograph: 1 cm on photo represents 100 m on actual ground.
Distance measured on photograph for the blue car = 2.0 cm
Time interval between the two photographs (t) = 10 s
Scale of photograph: 1 cm on photo represents 100 m on actual ground.
Distance measured on photograph for the blue car = 2.0 cm
Step 1: Calculate Actual Distance Covered:
Actual Distance (d) = 2.0 cm × 100 m/cm = 200 metres
Step 2: Calculate Speed in m/s:
Speed = Distance / Time = 200 m / 10 s = 20 m/s
Step 3: Convert Speed to km/h:
Speed = 20 × (18/5) = 4 × 18 = 72 km/h
Conclusion:
The speed of the fastest car is 20 m/s (or 72 km/h).
The speed of the fastest car is 20 m/s (or 72 km/h).
CBSE Marking Scheme 2026-27
Scale conversion to actual distance (200 m)1 Mark
Calculation of speed in m/s (20 m/s)1 Mark
Conversion to km/h (72 km/h)1 Mark
Q12
Figure 9.5 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster? Explain with scientific reasoning.
2 Marks
✅ CBSE Standard Answer
Answer: Vehicle A is moving faster than Vehicle B.
Scientific Justification:
- On a distance-time graph, the slope (steepness) of the curve represents the speed of the vehicle:
Slope = (Change in Distance) / (Change in Time) = Speed - In the given graph, line A is steeper (has a greater slope / angle with the horizontal time axis) compared to line B.
- For any given fixed time interval t, vehicle A covers a noticeably larger vertical distance than vehicle B. Therefore, vehicle A moves at a higher speed.
CBSE Marking Scheme 2026-27
Identification of Vehicle A as the faster moving vehicle1 Mark
Explanation linking graph slope to speed1 Mark
Q13
Which of the following distance-time graphs shows a truck moving with speed which is not constant? Explain how graph geometry identifies variable speed.
2 Marks
✅ CBSE Standard Answer
Answer: The graph with a curved line (non-linear graph) shows a truck moving with a non-constant (variable) speed.
Geometric and Physical Reasons:
- Constant Speed (Uniform Motion): Represented by a straight line inclined to the time axis because equal distances are covered in equal time intervals.
- Zero Speed (Rest): Represented by a straight horizontal line parallel to the time axis.
- Non-Constant Speed (Non-uniform Motion): Represented by a curved line where the slope continuously changes from moment to moment. A changing slope indicates that the distance covered per unit time is either increasing (acceleration) or decreasing (deceleration).
CBSE Marking Scheme 2026-27
Identification of the curved-line graph1 Mark
Explanation of changing slope representing changing speed1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: High-Speed Train Vande Bharat Telemetry
Railway Engineering Case: During trial runs of the semi-high-speed Vande Bharat Express between New Delhi and Varanasi (distance = 760 km), the train covered the first 160 km in 1 hour, the next 300 km in 2 hours, and the remaining 300 km in 3 hours.
(i) Calculate the average speed of the train over the entire journey in km/h.
(ii) Did the train maintain uniform motion throughout the journey? Justify.
(iii) Convert the overall average speed into SI units (m/s).
(i) Calculate the average speed of the train over the entire journey in km/h.
(ii) Did the train maintain uniform motion throughout the journey? Justify.
(iii) Convert the overall average speed into SI units (m/s).
(i) Average Speed:
Total Distance = 160 + 300 + 300 = 760 km
Total Time = 1 + 2 + 3 = 6 hours
Average Speed = 760 / 6 ≈ 126.67 km/h
(ii) Motion Type: Non-uniform motion. The speeds in the three legs were 160 km/h (leg 1), 150 km/h (leg 2), and 100 km/h (leg 3). Because the speed varied across sections, the motion was non-uniform.
(iii) SI Units Conversion:
126.67 × (5/18) = 633.35 / 18 ≈ 35.19 m/s
⚡ Assertion & Reason (HOTS)
Assertion (A): If the distance-time graph of a moving scooter is a straight line parallel to the time axis, the scooter is at complete rest.
Reason (R): The slope of a distance-time graph gives the speed of an object, and a horizontal line parallel to the time axis has a slope equal to zero.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): The slope of a distance-time graph gives the speed of an object, and a horizontal line parallel to the time axis has a slope equal to zero.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: When distance remains fixed as time elapses, Δd = 0. Therefore, Speed = Δd / Δt = 0. This translates graphically to zero slope (a horizontal line), proving that the vehicle is stationary.
10
Chapter 10: Electric Current and its Effects
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Circuit Fundamentals
- Symbols of Circuit Components:
- Electric Cell: Represented by two parallel lines — one long thin line (+ positive terminal) and one shorter thick line (- negative terminal).
- Battery: A combination of two or more cells connected such that the positive terminal of one cell connects to the negative terminal of the next.
- Switch: Shown open (OFF - incomplete circuit) or closed (ON - complete closed circuit).
- Electric Bulb: A circle enclosing an incandescent filament loop.
- Heating Effect of Electric Current (Joule Heating):
- When an electric current flows through a high-resistance wire, electrical energy is converted into heat energy. The wire becomes hot.
- Amount of heat produced depends on the wire's material, length, thickness, and the magnitude of current.
- Heating Appliances: Electric iron, room heater, geyser, toaster use coils of high-resistance wire called elements (usually made of Nichrome alloy).
- Electric Fuse: A safety device made of a special wire with a low melting point. If excessive current surges through the circuit due to overloading or short-circuiting, the fuse wire melts and breaks the circuit, preventing fires and device damage.
- MCB (Miniature Circuit Breaker): Modern electromagnetic/bimetallic switch that automatically trips OFF when current exceeds safe limits; easily reset without wire replacement.
- Magnetic Effect of Electric Current (Electromagnetism):
- Discovered in 1820 by Danish physicist Hans Christian Oersted, who noticed that a magnetic compass needle deflects whenever current passes through a nearby wire.
- Electromagnet: An insulated wire coil wound tightly around a soft iron core. It behaves like a magnet only when electric current flows through it. Magnetism disappears immediately when current is switched off.
- Applications: Scrap-metal cranes, electric bells, telephone receivers, MRI machines, maglev trains.
- Electric Bell: Operates on an electromagnet, an armature with a striker hammer, an adjustment contact screw, and a metallic gong in a self-breaking circuit loop.
NCERT Intext Questions & Scientific Enquiries
Page 124 — Intext Q1 (Paheli's Query)
Paheli asks: Why are Compact Fluorescent Lamps (CFLs) and Light Emitting Diodes (LEDs) preferred over traditional incandescent tungsten filament bulbs?
2 Marks
✅ CBSE Standard Answer
Energy Efficiency Comparison:
1. Wasted Thermal Energy in Incandescent Bulbs: An incandescent tungsten bulb functions by heating a filament to over 2500°C until it glows. Over 85–90% of electrical energy is wasted as heat, with only 10–15% converted into visible light.
2. Superior Efficiency of LEDs & CFLs: LEDs emit light via electroluminescence in semiconductor junctions, generating virtually zero excess heat. A 9-watt LED bulb produces the same brightness as a 60-watt incandescent bulb, saving over 80% electricity.
3. Longevity & Safety: LEDs last 25,000+ hours (compared to 1,000 hours for filament bulbs) and drastically reduce carbon footprints and electric utility bills.
1. Wasted Thermal Energy in Incandescent Bulbs: An incandescent tungsten bulb functions by heating a filament to over 2500°C until it glows. Over 85–90% of electrical energy is wasted as heat, with only 10–15% converted into visible light.
2. Superior Efficiency of LEDs & CFLs: LEDs emit light via electroluminescence in semiconductor junctions, generating virtually zero excess heat. A 9-watt LED bulb produces the same brightness as a 60-watt incandescent bulb, saving over 80% electricity.
3. Longevity & Safety: LEDs last 25,000+ hours (compared to 1,000 hours for filament bulbs) and drastically reduce carbon footprints and electric utility bills.
CBSE Marking Scheme 2026-27
Filament bulbs wasting 85-90% electricity as heat energy1 Mark
LEDs converting almost all electricity directly into light without heating1 Mark
Page 126 — Intext Q2 (Boojho's Query)
Boojho wants to know why a magnetic compass needle kept near an electric wire deflects ONLY when the switch is in the 'ON' position, but returns to normal North-South when 'OFF'.
2 Marks
✅ CBSE Standard Answer
Magnetic Effect of Current (Oersted's Principle):
1. When switch is 'OFF', no electric charges are flowing (I = 0). The wire has zero magnetic field. The compass needle responds only to Earth's natural geomagnetic field and points North-South.
2. When switch is moved to 'ON', electric current flows through the copper wire. Moving electrical charges generate an artificial magnetic field around the wire.
3. This magnetic field exerts a mechanical torque on the permanent magnetic compass needle, deflecting it away from North-South. When current is switched off, the magnetic field vanishes instantly, allowing the needle to swing back to Earth's North-South.
1. When switch is 'OFF', no electric charges are flowing (I = 0). The wire has zero magnetic field. The compass needle responds only to Earth's natural geomagnetic field and points North-South.
2. When switch is moved to 'ON', electric current flows through the copper wire. Moving electrical charges generate an artificial magnetic field around the wire.
3. This magnetic field exerts a mechanical torque on the permanent magnetic compass needle, deflecting it away from North-South. When current is switched off, the magnetic field vanishes instantly, allowing the needle to swing back to Earth's North-South.
CBSE Marking Scheme 2026-27
Moving electric current producing a magnetic field around the wire1 Mark
Magnetic interaction deflecting needle when ON; vanishing when OFF1 Mark
NCERT Textbook Exercise Questions
Q1
Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery.
3 Marks
✅ CBSE Standard Answer
| Electrical Component | Standard Circuit Symbol Description | Graphical Description / Symbol Representation |
|---|---|---|
| 1. Connecting Wire | Straight solid continuous line | ─── (A straight horizontal conductive line) |
| 2. Switch in 'OFF' position | Open key with gap (broken line) | ──o /o── (Two terminals with arm raised open) |
| 3. Electric Bulb | Loop inside a circle | A circle enclosing a coiled filament loop: ( ⌒ ) |
| 4. Electric Cell | Long thin line (+), short thick line (-) | ──| ▎── (Long line = positive, thick short line = negative) |
| 5. Switch in 'ON' position | Closed key (arm making contact) | ──o───o── (Both terminal dots bridged by solid line) |
| 6. Battery | Series combination of cell symbols | ──| ▎──| ▎──| ▎── (Long thin line followed by short thick line repeatedly) |
CBSE Marking Scheme 2026-27
Each correct electrical symbol drawn and labelled (0.5 Mark × 6)3 Marks
Q2
Draw the circuit diagram to represent the circuit shown in Fig. 10.21 (consisting of a single electric cell, a bulb, a switch in the ON position, and connecting wires).
2 Marks
✅ CBSE Standard Answer
A circuit diagram represents an electrical circuit using standardized schematic symbols rather than pictorial drawings of actual components.
Components to Connect in Series:
- Electric Cell: Draw a long thin line (+ve terminal) and a parallel shorter thick line (-ve terminal).
- Connecting Wire: Draw straight lines at right angles joining the components.
- Switch in 'ON' position: Draw two terminal dots bridged by a straight closed line (or a closed key).
- Electric Bulb: Draw a circle enclosing a filament loop. Since the switch is ON and the circuit is closed, add radiating lines around the bulb to depict glowing.
Schematic Trace:
Cell (+ terminal) ─── Connecting Wire ─── Switch (ON) ─── Electric Bulb (Glowing) ─── Connecting Wire ─── Cell (- terminal).
Cell (+ terminal) ─── Connecting Wire ─── Switch (ON) ─── Electric Bulb (Glowing) ─── Connecting Wire ─── Cell (- terminal).
CBSE Marking Scheme 2026-27
Correct use of all 4 standard schematic symbols in closed loop1 Mark
Neat rectangular layout and indication of glowing bulb1 Mark
Q3
Fig. 10.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.
2 Marks
✅ CBSE Standard Answer
To construct a battery of four cells, the cells must be connected in series such that the positive terminal (+, metallic cap) of one cell is connected to the negative terminal (-, flat zinc base) of the adjacent cell.
Wiring Sequence for 4 Cells (labelled 1, 2, 3, 4):
- Connect the negative terminal (-) of Cell 1 to the positive terminal (+) of Cell 2 using a connecting wire.
- Connect the negative terminal (-) of Cell 2 to the positive terminal (+) of Cell 3 using a connecting wire.
- Connect the negative terminal (-) of Cell 3 to the positive terminal (+) of Cell 4 using a connecting wire.
- The positive terminal (+) of Cell 1 and the negative terminal (-) of Cell 4 serve as the two free external terminals of the combined 4-cell battery to power the circuit.
Key Rule: Never connect positive to positive (+ to +) or negative to negative (- to -), as their opposing electromotive forces will cancel each other out and no current will flow.
CBSE Marking Scheme 2026-27
Correct opposite terminal (+ to -) series connection rule1 Mark
Clear wiring layout identifying two free external terminals1 Mark
Q4
The bulb in the circuit shown in Fig. 10.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.
2 Marks
✅ CBSE Standard Answer
Identification of the Problem:
In the given circuit, the two cells are connected incorrectly: the positive terminal of the first cell is connected to the positive terminal of the second cell (+ to +). Because similar polarity terminals oppose each other, the net voltage across the circuit is zero, preventing electric current from flowing.
In the given circuit, the two cells are connected incorrectly: the positive terminal of the first cell is connected to the positive terminal of the second cell (+ to +). Because similar polarity terminals oppose each other, the net voltage across the circuit is zero, preventing electric current from flowing.
Necessary Changes:
Turn one of the cells around so that the positive terminal (+, brass cap) of the first cell connects to the negative terminal (-, zinc base) of the second cell.
Once the cells are connected in series with opposite polarities, a complete closed circuit is formed and the bulb will glow brightly.
Turn one of the cells around so that the positive terminal (+, brass cap) of the first cell connects to the negative terminal (-, zinc base) of the second cell.
Once the cells are connected in series with opposite polarities, a complete closed circuit is formed and the bulb will glow brightly.
CBSE Marking Scheme 2026-27
Identification of identical polarities connected together (+ to +)1 Mark
Corrective measure (+ to - connection) to allow current flow1 Mark
Q5
Name any two effects of electric current.
1 Mark
✅ CBSE Standard Answer
Two major effects produced by electric current are:
1. Heating Effect of Electric Current: Conversion of electrical energy into thermal energy when current passes through a resistive conductor (e.g., electric iron, water geyser, room heater, fuse wire).
2. Magnetic Effect of Electric Current: Production of an invisible magnetic field around a current-carrying conductor (e.g., electromagnets, electric bells, electric motors).
(Note: Electric current also exhibits a Chemical Effect, e.g., electroplating).
CBSE Marking Scheme 2026-27
Listing Heating effect and Magnetic effect (0.5 Mark each)1 Mark
Q6
When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
2 Marks
✅ CBSE Standard Answer
Scientific Explanation:
- A compass needle is a tiny, freely pivoted permanent magnet that aligns itself along Earth's geomagnetic North-South axis when no other magnetic field is present.
- When electric current flows through the adjacent wire, the wire generates a magnetic field around itself (the magnetic effect of electric current, discovered by Hans Christian Oersted).
- This newly generated magnetic field exerts a mechanical magnetic force on the compass needle.
- The interaction between the magnetic field of the current-carrying wire and the permanent magnetic field of the compass needle forces the needle to deflect away from its natural North-South position.
CBSE Marking Scheme 2026-27
Mention of compass needle being a tiny magnet0.5 Mark
Current-carrying wire producing a magnetic field causing magnetic interaction1.5 Marks
Q7
Will the compass needle show deflection when the switch in the circuit shown by Fig. 10.24 is closed? Explain.
2 Marks
✅ CBSE Standard Answer
Answer: No, the compass needle will NOT show any deflection.
Scientific Reason:
- The circuit shown in Fig. 10.24 consists only of a bulb, connecting wires, and a switch — it lacks an electric source (no electric cell or battery).
- Without an electromotive power source (cell/battery), no potential difference exists to push electric charges through the circuit.
- Even when the switch is closed, no electric current flows through the wire (I = 0).
- Since there is no electric current, no magnetic field is generated around the wire. Therefore, the magnetic compass needle remains completely undeflected in its resting North-South alignment.
CBSE Marking Scheme 2026-27
Direct 'No' answer0.5 Mark
Absence of electric cell/battery leading to zero current and zero magnetic field1.5 Marks
Q8
Fill in the blanks:
(a) Longer line in the symbol for a cell represents its ______ terminal.
(b) The combination of two or more cells is called a ______.
(c) When current is switched ‘on’ in a room heater, it produces ______.
(d) The safety device based on the heating effect of electric current is called a ______.
(a) Longer line in the symbol for a cell represents its ______ terminal.
(b) The combination of two or more cells is called a ______.
(c) When current is switched ‘on’ in a room heater, it produces ______.
(d) The safety device based on the heating effect of electric current is called a ______.
2 Marks
✅ CBSE Standard Answer
(a) Longer line in the symbol for a cell represents its positive (+) terminal.
(b) The combination of two or more cells is called a battery.
(c) When current is switched ‘on’ in a room heater, it produces heat (due to red-hot nichrome heating elements).
(d) The safety device based on the heating effect of electric current is called a fuse.
CBSE Marking Scheme 2026-27
Each correct term correctly identified (0.5 Mark × 4)2 Marks
Q9
Mark ‘T’ if the statement is true and ‘F’ if it is false:
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
(c) An electromagnet does not attract a piece of iron. (T/F)
(d) An electric bell has an electromagnet. (T/F)
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
(c) An electromagnet does not attract a piece of iron. (T/F)
(d) An electric bell has an electromagnet. (T/F)
2 Marks
✅ CBSE Standard Answer
(a) False (F):
Reason: To form a functioning battery, the negative terminal of one cell must be connected to the positive terminal of the other cell.
Reason: To form a functioning battery, the negative terminal of one cell must be connected to the positive terminal of the other cell.
(b) True (T):
Reason: A fuse wire is fabricated from an alloy with a specific low melting point. Excess current generates excessive heat (H = I²Rt), melting the wire and isolating the circuit.
Reason: A fuse wire is fabricated from an alloy with a specific low melting point. Excess current generates excessive heat (H = I²Rt), melting the wire and isolating the circuit.
(c) False (F):
Reason: An electromagnet behaves like a strong magnet when energized and exerts a powerful magnetic attraction on ferromagnetic materials like iron and steel.
Reason: An electromagnet behaves like a strong magnet when energized and exerts a powerful magnetic attraction on ferromagnetic materials like iron and steel.
(d) True (T):
Reason: An electric bell incorporates an electromagnet (a U-shaped soft iron core wound with coils) to repeatedly attract the iron armature hammer.
Reason: An electric bell incorporates an electromagnet (a U-shaped soft iron core wound with coils) to repeatedly attract the iron armature hammer.
CBSE Marking Scheme 2026-27
Each correct evaluation with scientific justification (0.5 Mark × 4)2 Marks
Q10
Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
2 Marks
✅ CBSE Standard Answer
Answer: No, an electromagnet cannot be used to separate plastic bags from a garbage heap.
Scientific Reason:
- An electromagnet produces a magnetic field and can only attract magnetic substances such as iron, cobalt, nickel, and steel.
- Plastic is a completely non-magnetic polymer. It does not possess magnetic dipoles and is unaffected by magnetic lines of force.
- Therefore, an electromagnet will not exert any attractive force on plastic bags. Scrap heaps utilize electromagnets only to extract ferromagnetic scrap iron, not plastics.
CBSE Marking Scheme 2026-27
Clear 'No' assertion0.5 Mark
Explanation that magnets only attract magnetic substances, while plastic is non-magnetic1.5 Marks
Q11
An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.
3 Marks
✅ CBSE Standard Answer
Answer: No, I would strictly disagree and forbid the electrician from replacing the fuse with an ordinary piece of wire.
Scientific Reasons:
- Specific Low Melting Point: Standard fuse wire is manufactured from a specialized tin-lead alloy engineered with a low melting point and precise current rating.
- Ordinary Wires Do Not Melt: Ordinary connecting copper or aluminium wires have very high melting points and can withstand very high electrical currents without breaking.
- Severe Fire & Damage Hazard: If an electrical overload or short-circuit occurs, an ordinary copper wire will not melt. The heavy current will flow unchecked through household appliances, damaging expensive electronics, melting wall wiring insulation, and potentially causing an electrical building fire.
- Safe Alternative: Only an ISI-marked fuse cartridge of the correct rated amperage (e.g., 5 A or 15 A) or a Miniature Circuit Breaker (MCB) must be installed.
CBSE Marking Scheme 2026-27
Clear 'No' position0.5 Mark
Comparison of low melting point of fuse vs high melting point of ordinary wire1.5 Marks
Risk of appliance destruction and fire hazards1 Mark
Q12
Zubeda made an electric circuit using a cell holder, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.
3 Marks
✅ CBSE Standard Answer
The bulb may fail to glow due to any of the following technical defects in the circuit:
- Incorrect Cell Polarity: The cells in the cell holder may be placed backwards, connecting like terminals together (+ to + or - to -) instead of opposite terminals (+ to -).
- Exhausted / Discharged Cells: The chemical energy of the dry cells might be completely depleted (dead cells).
- Fused Bulb: The tungsten filament inside the bulb might be broken, resulting in an open circuit.
- Loose Connections: Connecting wires may not be tightly fastened to the terminals of the cell holder, switch, or bulb socket.
- Oxidation / Dirt on Terminals: Corrosion or rust on the metal contacts of the cell holder or switch contacts may introduce high contact resistance, blocking current flow.
- Faulty Switch: The switch contacts might not make proper conductive contact when moved to the 'ON' position.
CBSE Marking Scheme 2026-27
Any 3 well-explained possible defects (1 Mark each)3 Marks
Q13
In the circuit shown in Fig. 10.25 (with bulbs A, B, and C connected in series along with a switch and a cell):
(i) Would any of the bulbs glow when the switch is in the ‘OFF’ position?
(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
(i) Would any of the bulbs glow when the switch is in the ‘OFF’ position?
(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
2 Marks
✅ CBSE Standard Answer
(i) When switch is in 'OFF' position:
None of the bulbs will glow. When the switch is OFF, the electrical circuit is incomplete (open circuit). Electric current cannot flow across any part of an open circuit.
None of the bulbs will glow. When the switch is OFF, the electrical circuit is incomplete (open circuit). Electric current cannot flow across any part of an open circuit.
(ii) When switch is moved to 'ON' position:
All three bulbs (A, B, and C) will glow simultaneously at the exact same instant.
Scientific Justification: Electric current is the collective drift of free electrons present throughout the conducting wire loop. The moment the switch is closed, the electric field propagates through the circuit almost instantaneously at nearly the speed of light (~3 × 10⁸ m/s). Therefore, current starts flowing through every series component simultaneously, causing bulbs A, B, and C to illuminate together without any human-perceptible time delay.
All three bulbs (A, B, and C) will glow simultaneously at the exact same instant.
Scientific Justification: Electric current is the collective drift of free electrons present throughout the conducting wire loop. The moment the switch is closed, the electric field propagates through the circuit almost instantaneously at nearly the speed of light (~3 × 10⁸ m/s). Therefore, current starts flowing through every series component simultaneously, causing bulbs A, B, and C to illuminate together without any human-perceptible time delay.
CBSE Marking Scheme 2026-27
Part (i): Stating none of the bulbs glow due to open circuit1 Mark
Part (ii): Stating all bulbs glow simultaneously with scientific reasoning1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Modern Home Electrical Safety — Fuse vs MCB
Domestic Electrical Engineering: In an older apartment, a blown traditional porcelain fuse often plunged the house into darkness, requiring someone to search for fuse wire and rewire the cartridge in the dark. During renovation, an electrical engineer replaced all traditional fuses with Miniature Circuit Breakers (MCBs).
(i) On which two physical principles do MCBs operate to interrupt excessive current?
(ii) Why are MCBs considered far superior and safer than traditional rewirable fuses?
(iii) What should a homeowner inspect before resetting a tripped MCB back to the ON position?
(i) On which two physical principles do MCBs operate to interrupt excessive current?
(ii) Why are MCBs considered far superior and safer than traditional rewirable fuses?
(iii) What should a homeowner inspect before resetting a tripped MCB back to the ON position?
(i) Operating Principles: MCBs utilize two trip mechanisms:
1. Thermal effect (Bimetallic strip): For gradual sustained overloads, resistive heating causes a bimetallic strip to bend and mechanically release the latch.
2. Magnetic effect (Solenoid coil): For dangerous instantaneous short circuits, high surge current energizes an internal electromagnet plunger that instantly trips the contact open in milliseconds.
(ii) Advantages of MCBs:
• Quick Reset: Simply flip the toggle switch back up — no messy wire handling or risk of touching live terminals.
• Tamper-proof Safety: Calibrated factory ratings cannot be bypassed by using an oversized wire.
• High Sensitivity: Trips in 2.5 milliseconds, minimizing fire risk and appliance damage.
(iii) Safety Inspection: The homeowner must unplug the faulty appliance or rectify the underlying short circuit / overload that triggered the trip before flicking the MCB back ON, otherwise it will trip again instantly.
⚡ Assertion & Reason (HOTS)
Assertion (A): An electromagnet can be switched ON and OFF at will and its magnetic strength can be varied by adjusting the electric current.
Reason (R): The magnetic field of an electromagnet is produced by the electric current flowing through its solenoid coil, and soft iron cores lose their magnetism as soon as current stops.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): The magnetic field of an electromagnet is produced by the electric current flowing through its solenoid coil, and soft iron cores lose their magnetism as soon as current stops.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Unlike permanent magnets whose strength and poles are fixed, electromagnets are temporary magnets. Increasing current or number of coil turns amplifies magnetic flux, and cutting current drops magnetization to zero immediately, making them indispensable in heavy crane hoists and relays.
11
Chapter 11: Light
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Optics Fundamentals
- Rectilinear Propagation & Reflection of Light:
- Light travels in straight lines (rectilinear propagation). When light falls on a polished, shiny surface, its direction changes — a phenomenon known as reflection of light.
- Plane Mirror Image Characteristics:
- The image is erect (upright) and virtual (cannot be caught on a screen).
- The size of the image is identical to the object.
- The distance of the image behind the mirror is equal to the distance of the object in front of the mirror.
- The image exhibits lateral inversion: the left side of the object appears as the right side of the image, and vice versa.
- Spherical Mirrors:
- Concave Mirror (Converging): Inner curved surface is reflecting. Can form real, inverted images (diminished, same size, or magnified) when the object is beyond the focus; forms an erect, enlarged, and virtual image when placed very close (between pole and focus). Used by dentists, in solar furnaces, torch reflectors, and shaving mirrors.
- Convex Mirror (Diverging): Outer bulging surface is reflecting. Always forms an erect, virtual, and diminished (smaller) image over a very wide field of view. Used as rear-view mirrors in automobiles.
- Spherical Lenses:
- Convex Lens (Converging): Thicker at the middle than at edges. Converges incident parallel light rays. Can form real and inverted images, or a virtual, erect, and magnified image when held close to an object (used as a magnifying glass and in spectacles for hypermetropia).
- Concave Lens (Diverging): Thinner at the middle than at edges. Diverges incident parallel light rays. Always forms an erect, virtual, and diminished image regardless of object distance.
- Dispersion of Light & Sunlight:
- White sunlight consists of seven distinct constituent colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
- A glass prism splits white light into its seven constituent colours (dispersion of light), forming a rainbow spectrum.
- Newton's Disc: A circular disc painted with the seven rainbow segments rotated rapidly blends into a uniform white appearance, demonstrating that white light is a composite of the seven colours.
NCERT Intext Questions & Scientific Enquiries
Page 137 — Intext Q1 (Boojho's Query)
Boojho observed an emergency ambulance and wondered why the word AMBULANCE is written in reverse mirror-image lettering on the vehicle's front bonnet.
2 Marks
✅ CBSE Standard Answer
Optical Lateral Inversion Principle:
1. Plane and convex rear-view mirrors exhibit lateral inversion, meaning that an object's left side appears on the right side of the mirror image, and vice versa.
2. When the driver of a vehicle ahead glances into their rear-view mirror, the pre-inverted lettering on the ambulance bonnet undergoes lateral inversion in the mirror.
3. As a result, the reversed lettering appears right-side up as perfectly readable standard AMBULANCE. This allows drivers to instantly identify the approaching emergency vehicle and yield immediate right-of-way.
1. Plane and convex rear-view mirrors exhibit lateral inversion, meaning that an object's left side appears on the right side of the mirror image, and vice versa.
2. When the driver of a vehicle ahead glances into their rear-view mirror, the pre-inverted lettering on the ambulance bonnet undergoes lateral inversion in the mirror.
3. As a result, the reversed lettering appears right-side up as perfectly readable standard AMBULANCE. This allows drivers to instantly identify the approaching emergency vehicle and yield immediate right-of-way.
CBSE Marking Scheme 2026-27
Lateral inversion in driver's rear-view mirror reversing left and right1 Mark
Pre-inverted letters becoming standard readable text allowing prompt right-of-way1 Mark
Page 143 — Intext Q2 (Paheli's Query)
Paheli asks: Can we see the seven rainbow colours on a sunny day even without natural rainfall? How?
2 Marks
✅ CBSE Standard Answer
Artificial Dispersion of Sunlight:
Yes, rainbow colors can easily be produced and viewed without rainfall through any of the following optical methods:
Yes, rainbow colors can easily be produced and viewed without rainfall through any of the following optical methods:
- Water Fountain / Garden Hose: Stand with your back towards the sun and spray a fine mist of water droplets with a garden hose into the air. Tiny suspended water droplets act as micro-prisms, refracting, reflecting, and dispersing sunlight into a vibrant miniature rainbow.
- Glass Prism: Allow a narrow beam of sunlight through a window slit to pass through a triangular glass prism onto a white screen to see the 7 VIBGYOR colors.
- Soap Bubbles & Compact Discs: Thin films of soap bubbles or micro-grooves on the back of a CD reflect and disperse white light into shimmering rainbow spectral bands.
CBSE Marking Scheme 2026-27
Garden hose spray with sun behind observer creating rainbow droplets1 Mark
Dispersion via glass prism or soap bubble thin-film interference1 Mark
NCERT Textbook Exercise Questions
Q1
Fill in the blanks:
(a) An image that cannot be obtained on a screen is called ______.
(b) Image formed by a convex ______ is always virtual and smaller in size.
(c) An image formed by a ______ mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a ______ image.
(e) An image formed by a concave ______ cannot be obtained on a screen.
(a) An image that cannot be obtained on a screen is called ______.
(b) Image formed by a convex ______ is always virtual and smaller in size.
(c) An image formed by a ______ mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a ______ image.
(e) An image formed by a concave ______ cannot be obtained on a screen.
2.5 Marks
✅ CBSE Standard Answer
(a) An image that cannot be obtained on a screen is called a virtual image.
(b) Image formed by a convex mirror (or concave lens) is always virtual and smaller in size.
(c) An image formed by a plane mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a real image.
(e) An image formed by a concave lens cannot be obtained on a screen (because it is always virtual).
CBSE Marking Scheme 2026-27
Each correct blank accurately filled (0.5 Mark × 5)2.5 Marks
Q2
Mark ‘T’ if the statement is true and ‘F’ if it is false:
(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)
(b) A concave lens always forms a virtual image. (T/F)
(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)
(d) A real image cannot be obtained on a screen. (T/F)
(e) A concave mirror always forms a real image. (T/F)
(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)
(b) A concave lens always forms a virtual image. (T/F)
(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)
(d) A real image cannot be obtained on a screen. (T/F)
(e) A concave mirror always forms a real image. (T/F)
2.5 Marks
✅ CBSE Standard Answer
(a) False (F):
Reason: A convex mirror always forms an erect, virtual, and diminished (smaller) image; it can never produce an enlarged image.
Reason: A convex mirror always forms an erect, virtual, and diminished (smaller) image; it can never produce an enlarged image.
(b) True (T):
Reason: A concave lens is a diverging lens and always forms an erect, virtual, and diminished image for all object positions.
Reason: A concave lens is a diverging lens and always forms an erect, virtual, and diminished image for all object positions.
(c) True (T):
Reason: When an object is placed between the centre of curvature (C) and principal focus (F) of a concave mirror, it produces a real, inverted, and enlarged image beyond C.
Reason: When an object is placed between the centre of curvature (C) and principal focus (F) of a concave mirror, it produces a real, inverted, and enlarged image beyond C.
(d) False (F):
Reason: By definition, a real image is formed by actual converging light rays and can always be captured on a physical screen.
Reason: By definition, a real image is formed by actual converging light rays and can always be captured on a physical screen.
(e) False (F):
Reason: When an object is positioned very close to a concave mirror (between the pole and focus), it forms a virtual and erect image behind the mirror.
Reason: When an object is positioned very close to a concave mirror (between the pole and focus), it forms a virtual and erect image behind the mirror.
CBSE Marking Scheme 2026-27
Each correct evaluation with scientific rationale (0.5 Mark × 5)2.5 Marks
Q3
Match the items given in Column I with one or more items of Column II:
Column I:
(a) A plane mirror
(b) A convex mirror
(c) A convex lens
(d) A concave mirror
(e) A concave lens
Column II:
(i) Used as a magnifying glass.
(ii) Can form image of objects spread over a large area.
(iii) Used by dentists to see enlarged image of teeth.
(iv) The image is always inverted and magnified.
(v) The image is erect and of the same size than the object.
(vi) The image is erect and smaller in size than the object.
Column I:
(a) A plane mirror
(b) A convex mirror
(c) A convex lens
(d) A concave mirror
(e) A concave lens
Column II:
(i) Used as a magnifying glass.
(ii) Can form image of objects spread over a large area.
(iii) Used by dentists to see enlarged image of teeth.
(iv) The image is always inverted and magnified.
(v) The image is erect and of the same size than the object.
(vi) The image is erect and smaller in size than the object.
2.5 Marks
✅ CBSE Standard Answer
| Column I (Optical Device) | Column II (Matched Optical Properties / Application) |
|---|---|
| (a) A plane mirror | (v) The image is erect and of the same size as the object. |
| (b) A convex mirror | (ii) Can form image of objects spread over a large area (wide field of view), and (vi) The image is erect and smaller in size than the object. |
| (c) A convex lens | (i) Used as a magnifying glass (when object placed close to lens). |
| (d) A concave mirror | (iii) Used by dentists to see enlarged image of teeth (erect magnified virtual image). |
| (e) A concave lens | (vi) The image is erect and smaller in size than the object. |
CBSE Marking Scheme 2026-27
Each correct matching alignment (0.5 Mark × 5)2.5 Marks
Q4
State the characteristics of the image formed by a plane mirror.
3 Marks
✅ CBSE Standard Answer
The image formed by a flat plane mirror possesses the following distinct physical characteristics:
- Virtual Image: It is formed behind the mirror where reflected rays appear to diverge from, and cannot be captured or projected on a screen.
- Erect (Upright): The image stands right side up in the exact orientation as the object.
- Same Size: The height and width of the image are exactly identical to the dimensions of the object (Magnification m = +1).
- Equal Distance: The distance of the image behind the mirror surface is exactly equal to the distance of the object in front of the mirror (Image Distance = Object Distance).
- Laterally Inverted: The image exhibits lateral inversion, meaning the left side of the real object appears as the right side in the mirror image, and vice versa.
CBSE Marking Scheme 2026-27
Any 3 core characteristics correctly stated (1 Mark each)3 Marks
Q5
Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.
2 Marks
✅ CBSE Standard Answer
Symmetrical Capital Letters of English Alphabet:
The letters whose plane mirror image appears identical to the original letter itself are:
The letters whose plane mirror image appears identical to the original letter itself are:
A, H, I, M, O, T, U, V, W, X, Y
Discussion & Scientific Reason:
- Although every image in a plane mirror undergoes lateral inversion (reversal of left and right), these specific 11 letters possess a vertical line of reflective symmetry through their middle.
- Because their left half is the exact mirror image of their right half, swapping the left and right sides during mirror reflection does not produce any visible change in their overall appearance.
- Conversely, asymmetric letters like B, C, D, E, F, G, J, K, L, N, P, Q, R, S, Z do not have a vertical axis of symmetry, so their lateral inversion is immediately noticeable as reversed characters.
CBSE Marking Scheme 2026-27
Listing vertically symmetrical letters (A, H, I, M, O, T, U, V, W, X, Y)1 Mark
Scientific explanation based on vertical line symmetry1 Mark
Q6
What is a virtual image? Give one situation where a virtual image is formed.
2 Marks
✅ CBSE Standard Answer
Definition of Virtual Image:
A virtual image is an optical image formed when light rays originating from an object do not actually intersect at a physical point, but only appear to diverge from a location behind the reflecting or refracting surface. Consequently, a virtual image cannot be captured or obtained on a screen.
A virtual image is an optical image formed when light rays originating from an object do not actually intersect at a physical point, but only appear to diverge from a location behind the reflecting or refracting surface. Consequently, a virtual image cannot be captured or obtained on a screen.
Situations where a virtual image is formed:
- The everyday image of our face seen when looking into a plane mirror inside a dressing room.
- The smaller, upright rear view of following highway traffic observed in the convex side-view mirror of an automobile.
- The magnified upright letters viewed through a convex lens (magnifying glass) held close to a page.
CBSE Marking Scheme 2026-27
Clear definition (rays appear to diverge, cannot be cast on screen)1 Mark
One valid everyday situation (e.g. plane mirror or convex mirror)1 Mark
Q7
State two differences between a convex and a concave lens.
2 Marks
✅ CBSE Standard Answer
| Feature | Convex Lens (Converging Lens) | Concave Lens (Diverging Lens) |
|---|---|---|
| 1. Physical Geometry | It is thicker in the middle and thinner at its peripheral edges. | It is thinner in the middle and thicker at its peripheral edges. |
| 2. Action on Parallel Light Rays | It converges (brings together) parallel incoming light rays to a focal point. | It diverges (spreads out) parallel incoming light rays away from the axis. |
| 3. Nature of Images Formed | Can form both real (inverted) images and virtual (magnified, erect) images depending on distance. | Always forms an erect, virtual, and diminished (smaller) image for all positions. |
| 4. Common Application | Used as a magnifying glass, in microscopes, and cameras. | Used in peepholes of doors and spectacles for myopia. |
CBSE Marking Scheme 2026-27
Any 2 clear, accurate distinguishing differences (1 Mark each)2 Marks
Q8
Give one use each of a concave and a convex mirror.
2 Marks
✅ CBSE Standard Answer
1. Use of a Concave Mirror:
• Use: Used by dentists to examine teeth (or in automobile headlights and shaving mirrors).
• Scientific Reason: When held close to a tooth (within its focal length), a concave mirror forms an erect, enlarged (magnified), and clear virtual image, enabling the dentist to spot small cavities easily.
• Use: Used by dentists to examine teeth (or in automobile headlights and shaving mirrors).
• Scientific Reason: When held close to a tooth (within its focal length), a concave mirror forms an erect, enlarged (magnified), and clear virtual image, enabling the dentist to spot small cavities easily.
2. Use of a Convex Mirror:
• Use: Used as rear-view (wing) mirrors in automobiles (cars, buses, motorcycles).
• Scientific Reason: Convex mirrors always form an erect and diminished image, giving the driver a much wider field of view of traffic trailing behind than a flat plane mirror could ever provide.
• Use: Used as rear-view (wing) mirrors in automobiles (cars, buses, motorcycles).
• Scientific Reason: Convex mirrors always form an erect and diminished image, giving the driver a much wider field of view of traffic trailing behind than a flat plane mirror could ever provide.
CBSE Marking Scheme 2026-27
Use of concave mirror with scientific reason1 Mark
Use of convex mirror with scientific reason1 Mark
Q9
Which type of mirror can form a real image?
1 Mark
✅ CBSE Standard Answer
Answer: A Concave Mirror.
Scientific Explanation: Because a concave mirror is a converging spherical mirror, reflected light rays physically intersect in front of the mirror whenever the object is placed at or beyond the principal focus (F), thereby projecting a genuine real and inverted image that can be caught on a screen.
CBSE Marking Scheme 2026-27
Concave mirror identified with converging explanation1 Mark
Q10
Which type of lens forms always a virtual image?
1 Mark
✅ CBSE Standard Answer
Answer: A Concave Lens.
Scientific Explanation: A concave lens is a diverging lens that bends refracted light rays outwards away from the principal axis. These rays never converge in real space; when traced backwards, they appear to intersect, forming an image that is always virtual, erect, and diminished, irrespective of the object's distance.
CBSE Marking Scheme 2026-27
Concave lens named with diverging property1 Mark
Q11
Choose the correct option: A virtual image larger than the object can be produced by a:
(i) concave lens
(ii) concave mirror
(iii) convex mirror
(iv) plane mirror
(i) concave lens
(ii) concave mirror
(iii) convex mirror
(iv) plane mirror
1 Mark
✅ CBSE Standard Answer
Correct Option: (ii) concave mirror
Scientific Justification:
- A plane mirror always forms a virtual image of the exact same size as the object.
- A convex mirror always forms a virtual image that is diminished (smaller).
- A concave lens always forms a virtual image that is diminished (smaller).
- Only a concave mirror can produce an enlarged (magnified) virtual image when an object is held close to it (between its pole and focus).
CBSE Marking Scheme 2026-27
Correct option (ii) concave mirror with justification1 Mark
Q12
David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be:
(i) 3 m
(ii) 5 m
(iii) 6 m
(iv) 8 m
(i) 3 m
(ii) 5 m
(iii) 6 m
(iv) 8 m
2 Marks
✅ CBSE Standard Answer
Correct Option: (iii) 6 m
Detailed Step-by-Step Calculation:
- In a plane mirror, Object Distance = Image Distance from the mirror surface.
- Initially, the distance between the mirror and the image is 4 m. Hence, the initial distance of David (object) from the mirror was also 4 m.
- When David walks 1 m towards the mirror, his new distance from the mirror becomes:
New Object Distance = 4 m - 1 m = 3 m - Consequently, the new image will form 3 m behind the mirror:
New Image Distance = 3 m - The total distance between David and his image is the sum of object distance and image distance:
Total Distance = 3 m + 3 m = 6 m
CBSE Marking Scheme 2026-27
Calculation of new distance to mirror (3 m)1 Mark
Total separation between person and image (6 m)1 Mark
Q13
The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be:
(i) 1 m/s
(ii) 2 m/s
(iii) 4 m/s
(iv) 8 m/s
(i) 1 m/s
(ii) 2 m/s
(iii) 4 m/s
(iv) 8 m/s
2 Marks
✅ CBSE Standard Answer
Correct Option: (iii) 4 m/s
Physical & Relative Velocity Proof:
- The truck is stationary (parked). As the car reverses towards the truck at a speed of 2 m/s, the physical distance between the car's mirror and the parked truck decreases at a rate of 2 m/s.
- In a plane mirror, the distance of the image behind the mirror is always equal to the distance of the object from the mirror.
- Therefore, when the object distance decreases by 2 metres in 1 second, the image distance behind the mirror also decreases by 2 metres in that same 1 second.
- Hence, relative to the driver sitting in the moving car:
• The mirror moves 2 m closer to the truck.
• The image advances 2 m closer towards the mirror.
• The rate at which the image approaches the driver = 2 m/s + 2 m/s = 4 m/s.
CBSE Marking Scheme 2026-27
Selection of 4 m/s (Option iii)1 Mark
Clear relative velocity / optical doubling explanation1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Automobile Safety & "AMBULANCE" Inscription
Optical Application: On emergency response ambulances, the word AMBULANCE is painted in reverse lettering on the front bonnet.
(i) Which optical phenomenon of plane mirrors is this design based on?
(ii) How does this reverse lettering assist the driver in a car moving in front of the ambulance?
(iii) Why are convex mirrors, rather than plane mirrors, used for the wing/side-view mirrors in vehicles?
(i) Which optical phenomenon of plane mirrors is this design based on?
(ii) How does this reverse lettering assist the driver in a car moving in front of the ambulance?
(iii) Why are convex mirrors, rather than plane mirrors, used for the wing/side-view mirrors in vehicles?
(i) Optical Phenomenon: Lateral Inversion — the property of plane mirrors where left and right sides are interchanged.
(ii) Practical Benefit: When the driver of a vehicle ahead looks into their rear-view mirror, the laterally reversed word undergoes a second lateral inversion in the mirror, reading perfectly correctly as standard readable AMBULANCE. This allows the driver to immediately recognize the emergency and yield right-of-way without hesitation.
(iii) Reason for Convex Mirrors: Convex mirrors are curved outwards (diverging), which allows them to capture rays from a much wider angle. They produce upright, diminished images that give the driver a substantially broader panoramic field of view of trailing highway traffic compared to flat plane mirrors.
⚡ Assertion & Reason (HOTS)
Assertion (A): When white sunlight passes through a triangular glass prism, it emerges as a rainbow spectrum band of seven colours.
Reason (R): White light is a composite mixture of seven colours (VIBGYOR), each having a different speed and angle of bending (refraction) inside glass.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): White light is a composite mixture of seven colours (VIBGYOR), each having a different speed and angle of bending (refraction) inside glass.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Dispersion occurs because violet light slows down the most and bends through the greatest angle, while red light slows down the least and bends the least upon entering glass. This differential refraction spreads the composite beam into the visible spectrum band (VIBGYOR).
12
Chapter 12: Forests: Our Lifeline
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Forest Ecosystem Architecture
- Forest Structure & Stratification:
- Canopy: The uppermost continuous roof formed by the crowns of tall mature trees over the forest floor.
- Understorey: The distinct horizontal layers formed below the canopy, consisting of smaller trees, shrubs, and tall bushes.
- Forest Floor: Covered with dead leaves, twigs, seeds, animal waste, and herbs; home to millipedes, beetles, fungi, and decomposers.
- Ecosystem Balance & Food Chains:
- Forests are self-sustaining, dynamic living entities (a "super-organism").
- Food Chain: Energy flow from solar radiation to autotrophic producers (green plants) → primary consumers (herbivores like deer, grasshoppers) → secondary/tertiary consumers (carnivores like frogs, tigers) → decomposers.
- Decomposers: Micro-organisms (fungi, bacteria) that decompose dead organic matter into dark organic humus, recycling nutrients back into the soil for root uptake. There is zero waste in a virgin forest.
- Ecological & Hydrological Services:
- "Green Lungs" of the Earth: Forests absorb atmospheric carbon dioxide (CO₂) through photosynthesis and replenish oxygen (O₂), preserving atmospheric gas equilibrium.
- Natural Water Filter & Flood Barrier: Dense leaf canopies dissipate torrential rainfall impact, while extensive root networks bind the soil (preventing soil erosion) and facilitate rapid rainwater seepage into aquifers, maintaining the water table and preventing flash floods.
NCERT Intext Questions & Scientific Enquiries
Page 149 — Intext Q1
Why did birds and monkeys start making loud alarming noises and scampering from tree to tree as soon as children entered the forest fringe?
2 Marks
✅ CBSE Standard Answer
Ecological Warning System:
1. In a natural forest, arboreal animals like langurs, monkeys, and birds maintain high vigilance from tree crowns.
2. The entry of unfamiliar humans or potential predators is immediately perceived as a security threat.
3. Monkeys and birds produce specialized, loud vocal warning alarm calls that echo across the canopy, alerting deer, wild boars, and other ground-dwelling herbivores to seek cover and stay alert against danger.
1. In a natural forest, arboreal animals like langurs, monkeys, and birds maintain high vigilance from tree crowns.
2. The entry of unfamiliar humans or potential predators is immediately perceived as a security threat.
3. Monkeys and birds produce specialized, loud vocal warning alarm calls that echo across the canopy, alerting deer, wild boars, and other ground-dwelling herbivores to seek cover and stay alert against danger.
CBSE Marking Scheme 2026-27
High canopy vigilance of birds and monkeys detecting approaching intruders1 Mark
Alarm warning calls alerting ground herbivores to seek safety1 Mark
Page 153 — Intext Q2 (Boojho's Query)
Boojho asks: Why does rainwater not collect and form stagnant puddles on the forest floor, whereas city asphalt roads get flooded after a short shower?
2 Marks
✅ CBSE Standard Answer
Hydrological Sponge Action of Forests:
1. Interception & Gentle Drip: The multi-tiered canopy and understorey intercept raindrops, reducing their velocity so water drips gently onto the ground instead of slamming into it.
2. Spongy Organic Humus: The forest floor is covered with a thick carpet of decaying leaves, twigs, and crumbly humus that absorbs immense volumes of water like a giant sponge.
3. Deep Infiltration via Root Channels: Extensive root networks keep forest soil porous, facilitating rapid underground percolation into aquifers.
4. In contrast, impermeable concrete and asphalt roads in cities have zero porosity, forcing all rain to turn into immediate surface runoff and street flooding.
1. Interception & Gentle Drip: The multi-tiered canopy and understorey intercept raindrops, reducing their velocity so water drips gently onto the ground instead of slamming into it.
2. Spongy Organic Humus: The forest floor is covered with a thick carpet of decaying leaves, twigs, and crumbly humus that absorbs immense volumes of water like a giant sponge.
3. Deep Infiltration via Root Channels: Extensive root networks keep forest soil porous, facilitating rapid underground percolation into aquifers.
4. In contrast, impermeable concrete and asphalt roads in cities have zero porosity, forcing all rain to turn into immediate surface runoff and street flooding.
CBSE Marking Scheme 2026-27
Spongy humus layer absorbing water and facilitating percolation1 Mark
Roots creating porous soil channels vs impermeable concrete city roads1 Mark
NCERT Textbook Exercise Questions
Q1
Explain how animals dwelling in the forest help it grow and regenerate.
3 Marks
✅ CBSE Standard Answer
Animals living in a forest play crucial symbiotic roles that stimulate forest growth, seed dispersal, and continuous regeneration:
- Seed Dispersal: Herbivores and frugivorous birds feed on forest fruits. The indigestible seeds pass through their digestive tracts unharmed and are deposited far away with manure, allowing seedlings to sprout across new territories without overcrowding. Hooked seeds (like Xanthium) cling to animal fur and are carried to distant clearings.
- Pollination of Flowers: Flying insects (butterflies, bees, moths), birds, and bats visit flowers for nectar, transferring pollen grains between blossoms to ensure cross-pollination and viable seed formation.
- Nutrient Enrichment (Natural Manure): Animal excreta (dung, droppings) and decomposing carcasses provide essential organic matter. Microorganisms decompose these wastes into nutrient-rich humus, providing essential nitrogen, phosphorus, and potassium that fertilize germinating seedlings.
- Soil Aeration & Overgrowth Control: Burrowing animals (earthworms, ants, rodents) loosen the soil, improving root respiration and water infiltration, while grazing animals prevent aggressive undergrowth from suffocating young saplings.
CBSE Marking Scheme 2026-27
Role in seed dispersal with examples1 Mark
Role in cross-pollination1 Mark
Role of excreta/manure in forming humus for regeneration1 Mark
Q2
Explain how forests prevent floods.
3 Marks
✅ CBSE Standard Answer
Forests act as natural giant sponges and flood control buffers through the following hydrological mechanisms:
- Canopy Interception: The dense overhead roof of interlocking tree crowns and leaves intercepts torrential downpours, breaking the kinetic force of falling raindrops and preventing direct high-velocity impact on bare soil.
- Slow Drip to Forest Floor: Rainwater trickles slowly down tree trunks and drips gently onto the porous, litter-covered forest floor rather than surging over the surface.
- Natural Sponge & Percolation: The thick spongy layer of decaying leaves, humus, and organic mulch absorbs vast quantities of water, allowing it to slowly percolate deep underground into aquifers, steadily recharging the groundwater table.
- Prevention of Surface Runoff & Erosion: Extensive tree root networks bind the soil particles tightly, preventing them from being washed away (preventing siltation of rivers and reservoirs). By moderating surface runoff, forests ensure rivers receive a steady, controlled supply of water throughout the year rather than sudden catastrophic flash floods.
CBSE Marking Scheme 2026-27
Canopy breaking impact of heavy rain1 Mark
Forest floor acting as sponge aiding groundwater percolation1 Mark
Roots binding soil and regulating surface runoff to prevent flash floods1 Mark
Q3
What are decomposers? Name any two of them. What do they do in the forest?
3 Marks
✅ CBSE Standard Answer
1. Definition of Decomposers:
Decomposers are saprophytic micro-organisms that feed upon dead plant tissues, fallen leaves, rotten wood, and animal carcasses, breaking down complex organic molecules into simpler inorganic nutrients.
Decomposers are saprophytic micro-organisms that feed upon dead plant tissues, fallen leaves, rotten wood, and animal carcasses, breaking down complex organic molecules into simpler inorganic nutrients.
2. Examples of Decomposers:
• Bacteria
• Fungi (e.g., mushrooms, moulds)
• Bacteria
• Fungi (e.g., mushrooms, moulds)
3. Role in the Forest Ecosystem:
- They convert dead plant and animal remains into a dark, crumbly organic substance known as humus.
- Humus enriches the forest topsoil with essential mineral nutrients (nitrogen, carbon, potassium, phosphorus).
- These released mineral nutrients are subsequently re-absorbed by tree roots, maintaining a continuous cyclic flow of nutrients in the ecosystem.
- They prevent dead debris from piling up, serving as natural recyclers and cleaners of the forest floor.
CBSE Marking Scheme 2026-27
Precise scientific definition of decomposers1 Mark
Two valid examples (Bacteria, Fungi)0.5 Mark
Role in humus formation and recycling of mineral nutrients1.5 Marks
Q4
Explain the role of forest in maintaining the balance between oxygen and carbon dioxide in the atmosphere.
3 Marks
✅ CBSE Standard Answer
Forests are globally recognized as the "green lungs of nature" because of their indispensable role in maintaining the atmospheric equilibrium of oxygen and carbon dioxide:
- Photosynthesis (Oxygen Generation & Carbon Sink):
During daylight hours, trillions of leaves in a forest absorb immense volumes of carbon dioxide (CO₂) from the atmosphere to synthesize glucose, releasing massive quantities of life-sustaining oxygen gas (O₂) into the air:
6CO₂ + 6H₂O + Solar Energy → C₆H₁₂O₆ + 6O₂ - Respiration:
All forest animals, micro-organisms, and plants inhale oxygen for cellular respiration and release carbon dioxide back into the atmosphere:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy - Net Ecological Balance:
Because the net oxygen produced during daytime photosynthesis far exceeds the oxygen consumed during respiration by forest organisms, the surplus oxygen is dispersed globally to sustain animal life across the planet, while excess carbon dioxide is locked away in forest biomass, preventing runaway global warming.
CBSE Marking Scheme 2026-27
Explanation of photosynthesis consuming CO₂ and liberating O₂1.5 Marks
Explanation of respiration and net dynamic atmospheric balance ("green lungs")1.5 Marks
Q5
Explain why there is no waste in a forest.
2 Marks
✅ CBSE Standard Answer
Answer: There is no waste in a forest because all biological by-products are naturally recycled within closed ecological food chains and cycles.
Key Reasons:
- Scavengers Consume Carrion: Scavengers (vultures, crows, jackals, hyenas) quickly consume dead animal carcasses.
- Decomposers Complete Mineralisation: Any remaining organic matter, fallen dry leaves, animal excreta, and decaying logs are attacked by millions of decomposers (fungi, bacteria, beetles, earthworms).
- Transformation into Humus: Decomposers break down these organic residues into humus, releasing base minerals and nitrogen back into the topsoil.
- Root Uptake: Plants immediately reabsorb these mineral nutrients through their root hairs to build new leaves, branches, and fruits.
- Because everything produced is consumed and re-assimilated, the forest operates as a perfectly zero-waste circular ecosystem.
CBSE Marking Scheme 2026-27
Role of scavengers and decomposers1 Mark
Conversion into humus and re-absorption by plant roots (circular zero-waste loop)1 Mark
Q6
List five products we get from forests.
2.5 Marks
✅ CBSE Standard Answer
Five important economic and domestic products obtained from forests include:
- Timber and Firewood: High-grade structural wood (teak, sal, rosewood, deodar) used for furniture, buildings, boats, and paper pulp.
- Medicinal Herbs and Pharmaceuticals: Natural therapeutic plants (cinchona for quinine, neem, tulsi, amla, rauwolfia for blood pressure).
- Gums, Resins, and Turpentine: Industrial bonding agents, paints, varnishes, and aromatics extracted from tree bark.
- Honey and Wax: Wild forest honey harvested from wild honeybee hives, and natural beeswax used in cosmetics.
- Spices, Lac, and Catechu (Katha): Forest spices (cinnamon, cardamom), lac for sealing wax and bangles, and katha for betel leaves.
CBSE Marking Scheme 2026-27
Any 5 distinct forest products listed (0.5 Mark × 5)2.5 Marks
Q7
Fill in the blanks:
(a) The insects, butterflies, honeybees and birds help flowering plants in ______.
(b) A forest is a purifier of ______ and ______.
(c) Herbs form the ______ layer in the forest.
(d) The decaying leaves and animal droppings in a forest enrich the ______.
(a) The insects, butterflies, honeybees and birds help flowering plants in ______.
(b) A forest is a purifier of ______ and ______.
(c) Herbs form the ______ layer in the forest.
(d) The decaying leaves and animal droppings in a forest enrich the ______.
2 Marks
✅ CBSE Standard Answer
(a) The insects, butterflies, honeybees and birds help flowering plants in pollination.
(b) A forest is a purifier of air and water.
(c) Herbs form the lowest layer in the forest.
(d) The decaying leaves and animal droppings in a forest enrich the soil with nutrients (or humus).
CBSE Marking Scheme 2026-27
Each correct fill-in-the-blank (0.5 Mark × 4)2 Marks
Q8
Why should we worry about the conditions and issues related to forests far from us?
3 Marks
✅ CBSE Standard Answer
Even if forests are located hundreds of kilometres away from our urban cities, their degradation directly impacts global planetary survival in the following ways:
- Global Warming & Climate Change: Forests act as planetary carbon sinks. Widespread deforestation releases stored carbon, increasing atmospheric CO₂ concentrations and accelerating global global warming, unpredictable monsoon cycles, and melting polar glaciers.
- Oxygen Depletion: Large tropical forests (like the Amazon and Western Ghats) generate substantial shares of the world's oxygen. Their loss threatens breathable atmospheric oxygen quality.
- Disruption of Rainfall & Water Cycles: Forests contribute massive volumes of water vapour to cloud formation through transpiration. Destroying forests leads to severe continental droughts, erratic rainfall, and shrinking rivers.
- Downstream Flooding & Siltation: Deforested mountain watersheds allow unchecked torrents of rainwater to wash down fertile topsoil into distant urban rivers, causing catastrophic silt build-up, flash flooding, and drinking water shortages in faraway plains.
- Loss of Global Biodiversity: Over 80% of terrestrial wildlife lives in forests. Deforestation drives species to permanent extinction, destroying potential future sources of life-saving medical compounds.
CBSE Marking Scheme 2026-27
Any 3 well-reasoned ecological connections (1 Mark each)3 Marks
Q9
Explain why there is a need of variety of animals and plants in a forest.
3 Marks
✅ CBSE Standard Answer
A high variety of plants and animals (biodiversity) is essential for the ecological resilience, stability, and survival of a forest:
- Interconnected Food Chains and Food Webs: Different herbivores feed on different species of plants (grasses, tender leaves, bark, fruits). A greater variety of herbivores provides abundant prey for diverse carnivores. This intricate food web ensures that no single species overpopulates or starves.
- Enhanced Nutrient Regeneration: A diverse mixture of tree species sheds varying types of foliage and produces complex roots, supporting diverse communities of soil microbes and decomposers that replenish a broader spectrum of soil minerals.
- Pest & Disease Resistance: In a diverse forest, individual trees of the same species are separated by other species. This spatial separation prevents insect infestations and plant epidemics from spreading unchecked across the entire landscape.
- Habitat Stratification: Different animal species occupy distinct vertical zones (canopy, understorey, forest floor, underground burrows), enabling the forest to maximize energy utilization without lethal inter-species competition.
CBSE Marking Scheme 2026-27
Role of variety in maintaining complex food chains and webs1 Mark
Decomposition diversity and soil nutrient enrichment1 Mark
Pest resistance and ecological balance1 Mark
Q10
In Fig. 12.15, the artist has forgotten to put the labels and directions on the arrows. Mark the directions on the arrows and label the diagram using the following labels: clouds, rain, atmosphere, carbon dioxide, oxygen, plants, animals, soil, roots, water table.
3 Marks
✅ CBSE Standard Answer
Correct Arrow Directions & Label Descriptions for Exam Schematics:
- Clouds & Rain: Arrow pointing downwards from Clouds towards the forest trees and ground labelled Rain.
- Atmosphere: Surrounding air layer above the tree canopy.
- Carbon Dioxide (CO₂):
• Arrow pointing from atmosphere down into plants (absorbed during photosynthesis).
• Arrow pointing from animals up into atmosphere (released during respiration). - Oxygen (O₂):
• Arrow pointing from plants out into atmosphere (released during photosynthesis).
• Arrow pointing from atmosphere towards animals (inhaled for respiration). - Plants & Roots: Green foliage labelled Plants, with subterranean branching networks penetrating the upper soil layer labelled Roots.
- Soil & Water Table: The dark ground layer holding roots labelled Soil; a downward arrow showing percolation to the underground blue saturated layer labelled Water Table.
CBSE Marking Scheme 2026-27
Accurate arrow directions for gas exchange (CO₂ in, O₂ out)1.5 Marks
Accurate hydrological arrow directions (rain down, percolation to water table)1.5 Marks
Q11
Which of the following is not a forest product?
(i) Gum
(ii) Plywood
(iii) Sealing wax
(iv) Kerosene
(i) Gum
(ii) Plywood
(iii) Sealing wax
(iv) Kerosene
1 Mark
✅ CBSE Standard Answer
Correct Option: (iv) Kerosene
Scientific Reason:
- Gum is a direct natural exudate tapped from forest trees (such as acacia).
- Plywood is manufactured by gluing together thin veneers of forest timber.
- Sealing wax is derived from lac, a resinous secretion produced by tiny insects dwelling on forest host trees.
- Kerosene is a liquid hydrocarbon fuel obtained from the fractional distillation of underground crude petroleum (a fossil fuel), not a direct product harvested from forests.
CBSE Marking Scheme 2026-27
Selection of (iv) Kerosene with petroleum distinction1 Mark
Q12
Which of the following statements is not correct?
(i) Forests protect the soil from erosion.
(ii) Plants and animals in a forest are not dependent on one another.
(iii) Forests influence the climate and water cycle.
(iv) Soil helps forests to grow and regenerate.
(i) Forests protect the soil from erosion.
(ii) Plants and animals in a forest are not dependent on one another.
(iii) Forests influence the climate and water cycle.
(iv) Soil helps forests to grow and regenerate.
1 Mark
✅ CBSE Standard Answer
Statement that is NOT correct: (ii) Plants and animals in a forest are not dependent on one another.
Scientific Justification:
Plants and animals in a forest are deeply interdependent:
- Animals depend on plants for food, shelter, and atmospheric oxygen.
- Plants depend on animals for pollination of flowers, dispersal of seeds, supply of carbon dioxide for photosynthesis, and manure from animal wastes to enrich soil.
- Statements (i), (iii), and (iv) are all scientifically accurate ecological facts.
CBSE Marking Scheme 2026-27
Identification of statement (ii) with symbiotic explanation1 Mark
Q13
Micro-organisms act upon the dead plants to produce:
(i) sand
(ii) mushrooms
(iii) humus
(iv) wood
(i) sand
(ii) mushrooms
(iii) humus
(iv) wood
1 Mark
✅ CBSE Standard Answer
Correct Option: (iii) humus
Scientific Explanation: Decomposer micro-organisms (bacteria and fungi) secrete digestive enzymes that biochemically decompose complex plant cellulose and lignin into a dark, amorphous, nutrient-rich organic compound called humus, which elevates the fertility and moisture-retention capacity of forest topsoil.
CBSE Marking Scheme 2026-27
Selection of humus with biochemical decomposition explanation1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Deforestation & Flash Flood Catastrophe
Environmental Disaster Case: A hill slopes district cleared 500 hectares of virgin mountain forest for an infrastructure project. During the following monsoon, an intense rainfall event of 180 mm in 24 hours caused massive landslides, wiped out roads, and caused catastrophic flash flooding in river valleys 80 km downstream.
(i) What natural role of tree roots on the slope was lost due to deforestation?
(ii) Why did rainwater rush down into the river so rapidly instead of recharging aquifers?
(iii) How does the canopy of a forest protect the underlying soil from detachment?
(i) What natural role of tree roots on the slope was lost due to deforestation?
(ii) Why did rainwater rush down into the river so rapidly instead of recharging aquifers?
(iii) How does the canopy of a forest protect the underlying soil from detachment?
(i) Loss of Soil Binding: Tree root systems act like biological reinforcing mesh that mechanically binds soil particles, prevents slope destabilization, and holds millions of tonnes of earth firmly against gravitational shear stress.
(ii) Loss of Infiltration: Without the spongy forest floor (humus and leaf litter) to absorb water and facilitate gradual underground percolation, rain became immediate high-speed surface runoff, causing catastrophic downstream torrents.
(iii) Canopy Shielding: An intact forest canopy absorbs the terminal velocity kinetic energy of large falling raindrops. Instead of forcefully impacting and dislodging topsoil particles (splash erosion), water gently drips down leaves and branches onto the ground.
⚡ Assertion & Reason (HOTS)
Assertion (A): Despite huge quantities of fallen leaves, dead animals, and droppings, a virgin forest floor never accumulates waste.
Reason (R): Decomposers (bacteria and fungi) convert all dead organic matter into fertile dark humus, which is rapidly recycled back into growing plants.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Decomposers (bacteria and fungi) convert all dead organic matter into fertile dark humus, which is rapidly recycled back into growing plants.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Natural ecosystems operate on a perfect circular bio-economy. Microscopic decomposers and macro-scavengers recycle every gram of biomass into chemical nutrients, ensuring that nature generates zero waste.
13
Chapter 13: Wastewater Story
NCERT Exercise Solutions — Complete Step-by-Step Answers with CBSE Marking Scheme 2026-27
📌 Key Scientific Concepts & Wastewater Treatment Plant (WWTP)
- Sewage & Contaminants:
- Sewage: Liquid waste containing dissolved and suspended impurities called contaminants.
- Organic Impurities: Human excreta, animal waste, oil, urea, pesticides, herbicides, fruit/vegetable waste.
- Inorganic Impurities: Nitrates, phosphates, heavy metals.
- Nutrients: Phosphorus and Nitrogen.
- Bacteria & Microbes: Pathogens causing cholera (Vibrio cholerae), typhoid (Salmonella typhi), dysentery, and hepatitis.
- Wastewater Treatment Plant (WWTP) Stages:
- Physical Stage:
- Bar Screens: Screen out large floating debris like plastic bottles, rags, cans, sticks, and napkins.
- Grit & Sand Removal Tank: Wastewater flows slowly, allowing heavy sand, grit, and pebbles to settle down by gravity.
- Clarifier / Sedimentation Tank: Suspended solid faeces settle at the bottom as sludge (scraped off); lighter grease and oils float to the top and are removed by a skimmer. The output is clarified water.
- Biological Stage: Aeration Tank: Air is continuously pumped into clarified water to encourage the rapid growth of aerobic bacteria. These microbes consume remaining human waste, food particles, soaps, and organic impurities.
- Disinfection Stage: Treated water is disinfected using chemical agents like Chlorine or Ozone (O₃) or ultraviolet light before being safely discharged into natural rivers or the ocean.
- Physical Stage:
- Sludge & Biogas Utilization: Sludge from the clarifier is transferred to an anaerobic digester where anaerobic bacteria digest it, generating combustible biogas (used for electricity/cooking fuel). The remaining dried sludge forms nutrient-rich organic manure.
NCERT Intext Questions & Scientific Enquiries
Page 163 — Intext Q1 (Paheli's Query)
Paheli asks: Why should we never discard solid food scraps, used tea leaves, cotton, and sanitary items into the kitchen drain or toilet bowl?
2 Marks
✅ CBSE Standard Answer
Plumbing & Ecological Reasons:
1. Choking Drainage Conduits: Solid food scraps, viscous fats, tea leaves, and sanitary napkins do not dissolve in water. They agglomerate inside narrow pipe elbows and traps, trapping hair and debris to form stubborn blockages that cause foul sewer back-ups into homes.
2. Smothering Aerobic Microbes: In wastewater treatment systems, solid debris blankets bottom clarifiers, clogging intake pumps and reducing dissolved oxygen levels needed by aerobic bacteria to purify sewage.
Correct Practice: All solid organic refuse must be segregated and thrown into household dry/wet waste dustbins.
1. Choking Drainage Conduits: Solid food scraps, viscous fats, tea leaves, and sanitary napkins do not dissolve in water. They agglomerate inside narrow pipe elbows and traps, trapping hair and debris to form stubborn blockages that cause foul sewer back-ups into homes.
2. Smothering Aerobic Microbes: In wastewater treatment systems, solid debris blankets bottom clarifiers, clogging intake pumps and reducing dissolved oxygen levels needed by aerobic bacteria to purify sewage.
Correct Practice: All solid organic refuse must be segregated and thrown into household dry/wet waste dustbins.
CBSE Marking Scheme 2026-27
Solid materials not dissolving, causing severe drain pipe blockages1 Mark
Clogging treatment pumps and choking oxygen flow needed by purifying bacteria1 Mark
Page 165 — Intext Q2 (Boojho's Query)
Boojho asks: Why are eucalyptus trees planted along municipal wastewater treatment ponds and open sewage channels?
2 Marks
✅ CBSE Standard Answer
Phytoremediation / Bio-drainage by Eucalyptus:
1. High Transpiration Rate: Eucalyptus trees have deep root systems and extraordinarily high water-absorption rates, acting as natural biological water pumps.
2. Purification into Clean Water Vapor: The roots absorb surplus nutrient-rich wastewater rapidly, filtering out organic compounds to nourish tree biomass, and transpire massive volumes of clean, pure water vapor into the atmosphere.
3. Pollution & Odour Mitigation: By rapidly drying out waterlogged sewage lagoons, they prevent foul odor, eliminate stagnant mosquito-breeding swamps, and provide valuable timber without chemical expenses.
1. High Transpiration Rate: Eucalyptus trees have deep root systems and extraordinarily high water-absorption rates, acting as natural biological water pumps.
2. Purification into Clean Water Vapor: The roots absorb surplus nutrient-rich wastewater rapidly, filtering out organic compounds to nourish tree biomass, and transpire massive volumes of clean, pure water vapor into the atmosphere.
3. Pollution & Odour Mitigation: By rapidly drying out waterlogged sewage lagoons, they prevent foul odor, eliminate stagnant mosquito-breeding swamps, and provide valuable timber without chemical expenses.
CBSE Marking Scheme 2026-27
High water absorption and rapid transpiration releasing clean water vapor1 Mark
Drying out sewage lagoons, preventing mosquito breeding and water stagnation1 Mark
NCERT Textbook Exercise Questions
Q1
Fill in the blanks:
(a) Cleaning of water is a process of removing ______.
(b) Wastewater released by houses is called ______.
(c) Dried ______ is used as manure.
(d) Drains get blocked by ______ and ______.
(a) Cleaning of water is a process of removing ______.
(b) Wastewater released by houses is called ______.
(c) Dried ______ is used as manure.
(d) Drains get blocked by ______ and ______.
2 Marks
✅ CBSE Standard Answer
(a) Cleaning of water is a process of removing pollutants (or contaminants).
(b) Wastewater released by houses is called sewage.
(c) Dried sludge is used as manure.
(d) Drains get blocked by cooking oil (fats) and solid food remains (or plastics).
CBSE Marking Scheme 2026-27
Each blank accurately filled (0.5 Mark × 4)2 Marks
Q2
What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.
3 Marks
✅ CBSE Standard Answer
1. Definition of Sewage:
Sewage is wastewater carried away by a drainage network of sewers from homes, industries, agricultural fields, and public institutions. It is a complex liquid suspension containing dissolved organic and inorganic impurities, nutrients, toxic chemicals, and disease-causing microbes.
Sewage is wastewater carried away by a drainage network of sewers from homes, industries, agricultural fields, and public institutions. It is a complex liquid suspension containing dissolved organic and inorganic impurities, nutrients, toxic chemicals, and disease-causing microbes.
2. Hazards of Discharging Untreated Sewage into Water Bodies:
- Outbreak of Waterborne Diseases: Untreated sewage contains dangerous pathogens that contaminate drinking water sources, sparking deadly epidemics of cholera, typhoid, jaundice (hepatitis), dysentery, and polio.
- Severe Aquatic Oxygen Depletion (Eutrophication): Aerobic micro-organisms rapidly decompose organic wastes in sewage, consuming dissolved oxygen (DO). As oxygen plunges to near-zero levels, fish, crabs, and other aquatic life suffocate and die en masse.
- Destruction of Aquatic Ecosystems: Toxic household detergents, industrial chemicals, and heavy metals bioaccumulate in marine life, devastating coral reefs and marine food webs.
- Foul Odour & Environmental Degradation: Unchecked anaerobic decomposition emits toxic hydrogen sulphide (H₂S) and methane gases, making water bodies putrid, uninhabitable eyesores.
CBSE Marking Scheme 2026-27
Precise definition of sewage1 Mark
Explanation of waterborne epidemics and oxygen depletion in aquatic life2 Marks
Q3
Why should oils and fats not be released in the drain? Explain.
2 Marks
✅ CBSE Standard Answer
Cooking oils, fats, and grease should never be poured down kitchen sinks or household drains for two major reasons:
- Clogging of Drainage Pipes: As liquid oils and melted butter flow through cold underground plumbing pipes, they cool down and solidify into thick, sticky fat deposits. These deposits trap dirt and other debris, narrowing the pipe diameter and eventually causing complete, foul blockage of household drains.
- Choking of Open Soil Pores: In open drains and soil seepage channels, oily residues coat the soil particles, clogging natural microscopic soil pores. This prevents normal water percolation and suffocates the soil's natural capacity to filter wastewater.
Eco-friendly Disposal: Used cooking oil and solid grease should always be discarded into the solid garbage dustbin.
CBSE Marking Scheme 2026-27
Solidification and blocking of drainage pipes1 Mark
Clogging of soil pores reducing water filtration1 Mark
Q4
Describe the steps involved in getting clarified water from wastewater.
4 Marks
✅ CBSE Standard Answer
At a Wastewater Treatment Plant (WWTP), raw sewage is transformed into clarified water through the following sequential physical and biological stages:
Stage 1: Filtration through Bar Screens:
Wastewater first flows through large vertical bar screens. These screens intercept and strain out large floating physical debris such as plastic rags, polythene bags, sticks, cans, sanitary napkins, and pebbles.
Wastewater first flows through large vertical bar screens. These screens intercept and strain out large floating physical debris such as plastic rags, polythene bags, sticks, cans, sanitary napkins, and pebbles.
Stage 2: Grit and Sand Removal Chamber:
The water then passes through a grit and sand removal tank where the incoming flow velocity is deliberately slowed down. Heavy inorganic particles like coarse sand, gravel, and grit settle to the tank bottom by gravity and are mechanically removed.
The water then passes through a grit and sand removal tank where the incoming flow velocity is deliberately slowed down. Heavy inorganic particles like coarse sand, gravel, and grit settle to the tank bottom by gravity and are mechanically removed.
Stage 3: Primary Sedimentation (Clarifier Tank):
The water is channeled into a large sedimentation tank sloped downwards towards the centre. The water remains undisturbed for several hours:
The water is channeled into a large sedimentation tank sloped downwards towards the centre. The water remains undisturbed for several hours:
- Heavy solid organic wastes (human faeces) settle at the bottom as sludge and are removed by a bottom mechanical scraper.
- Light, floatable impurities such as oils and grease rise to the surface and are removed by a surface skimmer.
Stage 4: Aeration & Biological Purification:
Air is continuously pumped into the clarified water inside a massive aeration tank. The abundant oxygen triggers explosive growth of harmless aerobic bacteria, which consume residual dissolved organic impurities, food remnants, and soaps. After aeration, the microbes settle down as activated sludge, leaving clear, purified water ready for chemical disinfection.
Air is continuously pumped into the clarified water inside a massive aeration tank. The abundant oxygen triggers explosive growth of harmless aerobic bacteria, which consume residual dissolved organic impurities, food remnants, and soaps. After aeration, the microbes settle down as activated sludge, leaving clear, purified water ready for chemical disinfection.
CBSE Marking Scheme 2026-27
Stage 1: Bar screen coarse filtration1 Mark
Stage 2: Grit and sand chamber settling1 Mark
Stage 3: Primary sedimentation tank (sludge scraper + oil skimmer)1 Mark
Stage 4: Aeration tank aerobic bacterial breakdown1 Mark
Q5
What is sludge? Explain how it is treated.
3 Marks
✅ CBSE Standard Answer
1. Definition of Sludge:
Sludge is the solid, semi-liquid organic matter (primarily settled human faecal matter and particulate debris) that sinks and accumulates at the bottom of the primary sedimentation/clarifier tank during wastewater treatment.
Sludge is the solid, semi-liquid organic matter (primarily settled human faecal matter and particulate debris) that sinks and accumulates at the bottom of the primary sedimentation/clarifier tank during wastewater treatment.
2. Treatment of Sludge:
- Transfer to Anaerobic Digester: Sludge is continuously collected by mechanical scrapers and pumped into a sealed, oxygen-free chamber called an anaerobic digester.
- Anaerobic Digestion: Specialized anaerobic bacteria decompose the complex organic compounds in the sludge in the absence of oxygen.
- Biogas Production: This digestion process generates combustible biogas (a mixture of methane and carbon dioxide), which is captured and burned to generate electricity or used as fuel to power the treatment plant.
- Conversion to Organic Manure: After complete digestion, the remaining stabilized sludge is dehydrated on large sand drying beds. The dried solids yield an odorless, nutrient-rich organic manure that enriches agricultural soil.
CBSE Marking Scheme 2026-27
Definition of sludge as settled solid faecal matter1 Mark
Anaerobic decomposition producing biogas fuel1 Mark
Drying of digested residue into valuable agricultural manure1 Mark
Q6
Explain why untamed human excreta is a health hazard.
3 Marks
✅ CBSE Standard Answer
Untreated human excreta is a critical biological hazard for the following reasons:
- Severe Water Pollution: In open defecation areas, rain washes human faeces into nearby rivers, canals, ponds, and open wells. Furthermore, pathogens seep through porous soil into shallow aquifers, contaminating the groundwater that rural communities rely on for drinking and cooking.
- Transmission of Fatal Waterborne Diseases: A single gram of human faeces can contain millions of viruses, bacteria, and parasitic cysts. It is the primary vector for lethal waterborne epidemics, including:
- Cholera (caused by Vibrio cholerae)
- Typhoid (caused by Salmonella typhi)
- Dysentery & Diarrhoea
- Hepatitis A & Polio
- Vector Attraction (Houseflies & Pests): Exposed faecal mounds attract houseflies and cockroaches, which pick up microbial pathogens on their hairy legs and transfer them directly onto exposed human food, spreading infection.
- Soil Pollution & Worm Infections: Soil contaminated with faecal matter contains parasitic roundworm and hookworm eggs that penetrate barefoot walkers, causing chronic anaemia and malnutrition in children.
CBSE Marking Scheme 2026-27
Contamination of surface and groundwater resources1 Mark
Spread of waterborne pathogens (cholera, typhoid, hepatitis)1 Mark
Role of insect vectors and hookworm/parasitic infections1 Mark
Q7
Name two chemicals used to disinfect water.
1 Mark
✅ CBSE Standard Answer
Two chemicals widely used to disinfect water and destroy dangerous microscopic pathogens are:
1. Chlorine (added as chlorine gas, liquid sodium hypochlorite, or bleaching powder).
2. Ozone (O₃) (a powerful oxidant that sterilizes water without leaving residual chemical taste).
2. Ozone (O₃) (a powerful oxidant that sterilizes water without leaving residual chemical taste).
(Note: Ultraviolet (UV) radiation is also widely used as a physical disinfecting agent).
CBSE Marking Scheme 2026-27
Chlorine and Ozone correctly named (0.5 Mark each)1 Mark
Q8
Explain the function of bar screens in a wastewater treatment plant.
2 Marks
✅ CBSE Standard Answer
Function of Bar Screens:
- Physical Coarse Filtration: Bar screens act as the very first primary mechanical barrier at the entry point of a wastewater treatment plant.
- Interception of Bulky Objects: As raw sewage flows through the grid of parallel vertical metal bars, all large, floating physical debris — such as plastic polythene bags, fabric rags, empty cans, plastic bottles, sticks, and sanitary items — are physically caught and strained out.
- Protection of Downstream Equipment: By filtering out these large solids, bar screens prevent catastrophic clogging and damage to expensive high-pressure wastewater pumps, pipes, valves, and mechanical clarifier scrapers deeper within the facility.
CBSE Marking Scheme 2026-27
Screening out large solid floating debris (plastics, rags, cans)1 Mark
Protecting pumps and mechanical machinery from clogging and wear1 Mark
Q9
Explain the relationship between sanitation and disease.
3 Marks
✅ CBSE Standard Answer
Sanitation and human health share a direct, inverse relationship: poor sanitation breeds disease, whereas robust sanitation protects public health.
Key Mechanisms:
- Breeding Ground for Pathogens: Poor sanitation — such as open defecation, overflowing unsealed sewers, stagnant wastewater, and accumulated garbage — provides the ideal breeding environment for disease-causing bacteria, viruses, and parasites.
- Vector Proliferation: Stagnant wastewater puddles serve as breeding grounds for mosquitoes (transmitting malaria, dengue, and chikungunya), while open excreta attracts houseflies that transfer microbes onto food.
- Contamination of Drinking Supplies: In areas with inadequate sanitation, untreated sewage leaches into shallow water tables and pipelines, triggering outbreaks of fatal waterborne illnesses like cholera, dysentery, and typhoid.
- Sanitation Breaks the Chain of Infection: Proper sanitation — including sanitary toilets, underground closed sewage systems, clean drinking water, and hygienic handwashing — breaks the faecal-oral transmission route, eradicating infectious diseases and saving millions of children's lives annually.
CBSE Marking Scheme 2026-27
Direct relationship: Poor sanitation causes infectious diseases1 Mark
Contamination pathways via vectors and contaminated water1 Mark
Proper sanitation breaking the transmission cycle1 Mark
Q10
Outline your role as an active citizen in relation to sanitation.
3 Marks
✅ CBSE Standard Answer
As a responsible and proactive citizen, one should maintain civic hygiene and sanitation through the following concrete actions:
- Responsible Household Waste Disposal: Never pour cooking oils, paints, grease, medicines, or chemicals down household sinks. Dispose of solid tea leaves, cotton, food scraps, and plastic wrappers in designated dry/wet dustbins rather than drains.
- Reporting Open Sewer Hazards: If an open sewer, choked manhole, or broken municipal water pipe is spotted in the neighbourhood, immediately alert the local municipal corporation or Gram Panchayat to avert disease outbreaks.
- Eradicating Open Defecation: Promote and use sanitary closed toilets; advocate against open defecation and urination in open public spaces.
- Community Cleanliness Campaigns: Actively participate in neighbourhood sanitation drives (like the Swachh Bharat Abhiyan), ensuring stagnant rainwater puddles are sprayed or drained to curb mosquito breeding.
- Spreading Awareness: Educate family, neighbours, and community workers about proper hand hygiene, water purification, and the health benefits of covered sewage networks.
CBSE Marking Scheme 2026-27
Proper domestic disposal habits (no oils/solids down the drain)1 Mark
Civic reporting of leaks/choked manholes to municipal authorities1 Mark
Promoting community toilet use and hygiene education1 Mark
Q11
Solve the NCERT Crossword Puzzle on Wastewater Treatment terms:
Across Clues:
3. Liquid waste products.
4. Solid waste extracted in sewage treatment.
6. A word related to hygiene.
8. Waste matter discharged from human body.
Down Clues:
1. Used water.
2. A pipe carrying sewage.
5. Micro-organisms which cause cholera.
7. A chemical to disinfect water.
Across Clues:
3. Liquid waste products.
4. Solid waste extracted in sewage treatment.
6. A word related to hygiene.
8. Waste matter discharged from human body.
Down Clues:
1. Used water.
2. A pipe carrying sewage.
5. Micro-organisms which cause cholera.
7. A chemical to disinfect water.
4 Marks
✅ CBSE Standard Answer
| Direction & Clue Number | Clue Description | Crossword Solution Word |
|---|---|---|
| Across 3 | Liquid waste products | SEWAGE |
| Across 4 | Solid waste extracted in sewage treatment | SLUDGE |
| Across 6 | A word related to hygiene | SANITATION |
| Across 8 | Waste matter discharged from human body | EXCRETA |
| Down 1 | Used water | WASTEWATER |
| Down 2 | A pipe carrying sewage | SEWER |
| Down 5 | Micro-organisms which cause cholera | BACTERIA |
| Down 7 | A chemical to disinfect water | OZONE (or CHLORINE) |
CBSE Marking Scheme 2026-27
Each correct crossword term accurately mapped (0.5 Mark × 8)4 Marks
Q12
Study the following statements about ozone:
(a) It is essential for breathing of living organisms.
(b) It is used to disinfect water.
(c) It absorbs ultraviolet rays.
(d) Its proportion in air is about 3%.
Which of these statements are correct?
(i) (a), (b) and (c)
(ii) (b) and (c)
(iii) (a) and (d)
(iv) All four
(a) It is essential for breathing of living organisms.
(b) It is used to disinfect water.
(c) It absorbs ultraviolet rays.
(d) Its proportion in air is about 3%.
Which of these statements are correct?
(i) (a), (b) and (c)
(ii) (b) and (c)
(iii) (a) and (d)
(iv) All four
2 Marks
✅ CBSE Standard Answer
Correct Option: (ii) (b) and (c)
Scientific Evaluation of Each Statement:
- (a) Incorrect: Oxygen (O₂), not ozone (O₃), is essential for cellular respiration. Ozone is actually toxic and irritating to animal lungs if inhaled in significant quantities.
- (b) Correct: Ozone is a potent germicide and powerful oxidizing agent used in municipal waterworks to disinfect and sterilize water.
- (c) Correct: The stratospheric ozone layer shields Earth's biosphere by absorbing harmful ultraviolet (UV-B) solar radiation.
- (d) Incorrect: Ozone is a trace gas making up less than 0.00006% of Earth's atmosphere, nowhere near 3%.
CBSE Marking Scheme 2026-27
Correct option selection: (ii) (b) and (c)1 Mark
Accurate scientific justification of correct and incorrect statements1 Mark
🎯 Competency-Based Questions (CBQ) & HOTS
CBSE Board Exam Pattern
Case Study: Vermi-Processing Toilets & Low-Cost On-Site Sanitation
Biotechnology in Sanitation: In a remote rural village without underground sewerage infrastructure, an NGO introduced vermi-processing toilets. Human excreta is collected in a pit populated by red earthworms (tiger worms).
(i) What do the earthworms convert human excreta into?
(ii) Why are vermi-processing toilets considered ideal for rural communities with low water availability?
(iii) Name another traditional on-site sewage disposal system commonly used for isolated rural homes.
(i) What do the earthworms convert human excreta into?
(ii) Why are vermi-processing toilets considered ideal for rural communities with low water availability?
(iii) Name another traditional on-site sewage disposal system commonly used for isolated rural homes.
(i) Bio-conversion: Red worms ingest and thoroughly decompose human faeces, transforming pathogenic excreta into high-grade, pathogen-free vermicompost (vermi-cakes), which serves as nutrient-dense organic fertilizer for crops.
(ii) Advantages:
• Low Water Requirement: Requires minimal flushing water compared to standard cistern toilets.
• Simple & Zero Odour: Operates naturally without noxious odours, complex machinery, or expensive electrical connections.
• Resource Recovery: Directly converts hazardous waste into economic fertilizer right at the site.
(iii) Other On-site Systems: Septic tanks, biogas composting pits, and chemical toilets.
⚡ Assertion & Reason (HOTS)
Assertion (A): Cooking oils and solid food wastes should never be flushed down kitchen sinks into domestic drainage pipes.
Reason (R): Fats and oils solidify inside cold plumbing conduits, trapping food scraps and causing stubborn drain blockages, while choking soil filtration pores in open drains.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Reason (R): Fats and oils solidify inside cold plumbing conduits, trapping food scraps and causing stubborn drain blockages, while choking soil filtration pores in open drains.
Choose the correct option:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Scientific Explanation: Greasy hydrophobic lipids adhere to the walls of drainage pipes and harden as temperature drops, drastically constricting water flow. Discarding oils into solid waste receptacles preserves sewer line integrity.