Class 9 Science — NCERT Exercise Solutions

Classification:
• Matter: Chair, air, almonds, lemon water, smell of perfume.
Justification: These entities possess a measurable mass and occupy physical volume. (Note: While 'smell' as a pure sensation is not matter, the 'smell of perfume' represents volatile physical odorant particles/molecules dispersed in the air that enter our olfactory receptors, hence it is physical matter).

• Not Matter: Love, hate, thought, cold.
Justification: These represent human emotions, thoughts, feelings, or temperature sensations. They do not have mass and do not occupy physical space.

Scientific Explanation:
1. The particles of aroma (gaseous vapours) released from food diffuse through air molecules to reach our nose.
2. The rate of diffusion of particles is directly proportional to temperature. At higher temperatures, the kinetic energy of particles increases significantly (KE ∝ T).
3. In hot sizzling food, the particles have high kinetic energy, move at high speeds, and diffuse rapidly across large distances in air in a fraction of a second.
4. In cold food, the kinetic energy of particles is very low. Consequently, the rate of vaporization and diffusion is minimal, requiring us to go close to detect the smell.

Observation & Properties Demonstrated:
This observation demonstrates two fundamental characteristics of particles of liquids:

1. Presence of Intermolecular Spaces: The particles of liquid (water) have relatively large spaces between them compared to solids. This allows external objects like a diver's body to displace water particles and move through them.
2. Intermediate Force of Attraction: The intermolecular forces of attraction between water molecules are weak enough to be easily overcome by the muscular force exerted by the diver.

Fundamental Characteristics of Particles of Matter:

1. Particles of matter have spaces between them: When substances like sugar, salt, or potassium permanganate dissolve in water, their particles get accommodated into the empty inter-particle spaces between water molecules without increasing overall volume.
2. Particles of matter are continuously moving: Particles possess kinetic energy which increases with rise in temperature. This continuous random motion causes diffusion and Brownian movement.
3. Particles of matter attract each other: An attractive cohesive force holds particles together. The strength of this force is maximum in solids, intermediate in liquids, and minimum in gases.
4. Particles of matter are extremely small: Particles are unimaginably minute, far beyond our microscopic visibility. A single tiny crystal of potassium permanganate contains millions of tiny particles that can tint litres of water.

Scientific Analysis of Density:
Density (ρ = Mass / Volume) depends on mass of particles and how tightly packed they are in a given volume.
• Gases have lowest density (air, chimney exhaust containing hot gases and soot particles).
• Liquids have intermediate density (water, honey which is viscous and denser than water).
• Solids have highest density (cotton with trapped air is least dense solid, chalk, iron has most tightly packed atoms).

Order of Increasing Density:
Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron

(a) Differences between States of Matter:

Characteristic	Solid	Liquid	Gas
Shape	Definite fixed shape	No fixed shape (takes shape of container)	No fixed shape
Volume	Definite fixed volume	Definite fixed volume	No fixed volume (expands)
Intermolecular Space	Very closely packed (minimum)	Loosely packed (intermediate)	Very large spaces (maximum)
Intermolecular Force	Strongest	Intermediate	Negligible / weakest
Compressibility	Incompressible	Negligible compressibility	Highly compressible
Fluidity / Rigidity	Rigid, cannot flow	Fluid, flows from higher to lower	Fluid, flows in all directions

(b) Detailed Comments on Properties:
• Rigidity: Tendency of a substance to maintain its shape when subjected to external force. Solids are highly rigid.
• Compressibility: The property of decreasing volume when external pressure is applied. Gases are highly compressible because of large inter-particle voids.
• Fluidity: The ability of a substance to flow. Liquids and gases are fluids.
• Filling a gas container: Due to negligible intermolecular attraction and high kinetic energy, gas particles move in all directions and occupy the entire container.
• Shape: Structural geometry. Solids have fixed boundaries; liquids and gases adopt container shape.
• Kinetic Energy: Energy possessed due to particle motion. Gases have maximum KE, solids minimum.
• Density: Mass per unit volume (M/V). Generally, Solid > Liquid > Gas.

(a) Why gas fills container completely:
The intermolecular forces of attraction between gas molecules are negligible, and particles possess very high kinetic energy. Hence, gas particles move randomly at high speeds in all possible directions and distribute themselves uniformly, filling the vessel completely.

(b) Why gas exerts pressure on container walls:
Due to continuous random high-speed motion, gas particles constantly collide with one another and bombard the inner walls of the container. The force exerted by these particles per unit area of the wall constitutes gas pressure (P = F/A).

(c) Why a wooden table is called a solid:
A wooden table has a definite fixed shape, distinct boundaries, fixed volume, high rigidity, negligible compressibility, and cannot flow. These are all quintessential defining properties of a solid.

(d) Moving hand in air vs wood:
In air (gas), particles are widely separated with negligible intermolecular forces; our hand easily displaces air molecules. In a block of wood (solid), atoms are tightly bound by extremely strong cohesive forces with no free space. To overcome these intense binding forces, a massive localized force (like a karate strike) is required.

Anomalous Behavior and Cage-like Structure of Ice:
1. In liquid water, molecules are constantly moving and relatively compact.
2. When water freezes at 0°C (273 K) to form ice, water molecules arrange themselves into an open, tetrahedral three-dimensional crystal lattice held by extensive intermolecular hydrogen bonding.
3. This crystal lattice contains a large amount of empty space (cage-like structure), making the volume of ice greater than the volume of an equal mass of liquid water.
4. Since density is inversely proportional to volume (Density = Mass / Volume), the density of ice is lower than the density of liquid water (0.917 g/cm³ < 1.0 g/cm³).
5. Any substance with lower density than a liquid floats on it. Hence, ice floats on water.

(a) 300 K:
T(°C) = 300 - 273 = 27°C.

(b) 573 K:
T(°C) = 573 - 273 = 300°C.

(a) At 250°C:
The boiling point of pure water under standard atmospheric pressure is 100°C. Since 250°C is far above the boiling point, water exists entirely in the gaseous state (water vapour / steam).

(b) At 100°C:
100°C is the boiling point (transition temperature) of water. At this temperature, liquid water absorbs latent heat of vaporization to convert into steam.
Therefore, water exists in a dynamic equilibrium in both liquid and gaseous states (liquid water and steam coexist).

Concept of Latent Heat:
1. When heat is supplied to a substance at its melting point or boiling point, the temperature does not rise because the supplied thermal energy is entirely absorbed as Latent Heat ('hidden heat').
2. Instead of increasing the kinetic energy of particles (which would raise temperature), this energy is utilized to overcome and break the strong intermolecular forces of attraction holding the particles together in that state.
3. Once the phase transformation is 100% complete, any further supplied heat begins to increase the kinetic energy of particles again, causing the temperature to rise.

Method to Liquefy Gases:
Atmospheric gases can be liquefied by a combination of two processes:

1. Applying High Pressure: Compressing the gas brings its widely spaced molecules closer together, reducing intermolecular distances so cohesive forces can take hold.
2. Lowering Temperature (Cooling): Reducing temperature decreases the kinetic energy of gas molecules, slowing them down so they cannot resist intermolecular attractive forces.

Scientific Explanation:
1. A desert cooler operates on the principle of evaporative cooling (evaporation causes cooling).
2. On a hot dry day:
• High temperature provides abundant thermal energy, accelerating the rate of evaporation.
• Low humidity (dry air) means air can hold much more moisture, dramatically increasing the evaporation rate of water from cooler pads.
3. As water evaporates rapidly, it absorbs large quantities of latent heat of vaporization from the incoming air, lowering the air temperature significantly.
4. Hence, a desert cooler cools far more effectively on a hot, dry day than on a humid day.

Evaporative Cooling in Porous Clay Pots:
1. An earthen pot (matka) has millions of extremely fine microscopic pores on its clay surface.
2. Water seeps out continuously through these pores to the outer surface of the pot.
3. This thin film of water on the outer wall evaporates into the surrounding warm summer air.
4. For evaporation to occur, the water molecules absorb the required latent heat of vaporization from the pot and the remaining water inside.
5. As heat is continually extracted from the internal water, its temperature drops, making the water pleasantly cold.

Volatile Liquids & Latent Heat Absorption:
1. Acetone, petrol, and alcohol-based perfumes are highly volatile organic liquids with very low boiling points.
2. When placed on our palm, their particles evaporate very rapidly.
3. During this rapid evaporation, the molecules absorb the required latent heat of vaporization directly from our skin (palm).
4. Due to this loss of thermal energy, the temperature of our palm drops locally, producing a cooling sensation.

Surface Area Effect on Evaporation:
1. Evaporation is a surface phenomenon; its rate is directly proportional to the exposed surface area of the liquid.
2. A saucer has a significantly larger exposed surface area compared to a cylindrical tea cup.
3. When hot tea or milk is poured into a saucer, the increased surface area accelerates the rate of evaporation tremendously.
4. More evaporation causes rapid loss of latent heat, allowing the hot tea to cool down much faster and be sipped easily without burning the mouth.

Recommended Clothing: Light-coloured Cotton Clothes.
Scientific Reasons:

1. Sweat Absorption: During hot summers, our body perspires heavily to regulate internal temperature. Cotton is a natural, porous fabric and a good absorber of water; it quickly absorbs sweat and exposes it to the atmosphere.
2. Evaporative Cooling: As sweat evaporates from the surface of the cotton fabric, it absorbs latent heat of vaporization from our body, keeping us cool and comfortable.
3. Reflection of Heat: Light-coloured (white) clothes reflect solar thermal radiation rather than absorbing it, minimizing heat gain.

Formula: °C = K - 273
(a) 293 K = 293 - 273 = 20°C
(b) 470 K = 470 - 273 = 197°C

Formula: K = °C + 273
(a) 25°C = 25 + 273 = 298 K
(b) 373°C = 373 + 273 = 646 K

(a) Sublimation of Naphthalene:
Naphthalene undergoes sublimation — the direct phase change from solid to gas without passing through the intermediate liquid state.
At ambient room temperature, the intermolecular cohesive forces between naphthalene molecules are weak. They slowly and steadily vaporize into gaseous naphthalene molecules that disperse into the air, leaving behind zero solid residue.

(b) Diffusion of Perfume Vapours:
Perfumes contain highly volatile aromatic liquids. When sprayed, these volatile particles immediately vaporize into gas. Due to their high kinetic energy and large inter-particle spaces in air, the perfume molecules rapidly diffuse across the room through continuous random collisions with air molecules, reaching our nostrils several metres away.

Concept: Intermolecular force of attraction is strongest in solids, intermediate in liquids, and weakest in gases.
• Oxygen is a gas (negligible attraction between O₂ molecules).
• Water is a liquid (moderate hydrogen bonding).
• Sugar is a crystalline solid (strong intermolecular bonds).

(a) At 25°C:
25°C is standard room temperature, between the melting point (0°C) and boiling point (100°C). Water exists as a liquid.

(b) At 0°C:
0°C is the freezing/melting point of water. Both solid ice and liquid water coexist in equilibrium as latent heat of fusion is absorbed or released. Water exists as both solid and liquid.

(c) At 100°C:
100°C is the boiling point of water. Water exists as both liquid and gas (vapour / steam) in equilibrium as latent heat of vaporization is absorbed.

(a) Water is a liquid at room temperature because:

1. Water has a fixed definite volume but no fixed shape; it takes the shape of the container in which it is placed.
2. Water can flow readily (fluidity) and possesses negligible compressibility with moderate intermolecular forces of attraction.

(b) An iron almirah is a solid at room temperature because:

1. It possesses a fixed definite shape and fixed volume with rigid, distinct boundaries that do not deform under normal forces.
2. It is extremely rigid and practically incompressible due to strong metallic bonds and negligible inter-particle spaces.

Role of Latent Heat of Fusion:
1. Both ice and water at 273 K (0°C) are at the exact same temperature.
2. However, water at 273 K possesses an additional 3.34 × 10⁵ J/kg of heat energy in the form of latent heat of fusion compared to ice.
3. When ice at 273 K is placed in contact with a warm substance, it first absorbs this extra latent heat of fusion (3.34 × 10⁵ J/kg) from the substance just to melt into water at 0°C, and then absorbs further heat as its temperature rises.
4. Since ice extracts substantially more heat energy per unit mass from the surrounding medium, it is significantly more effective at cooling than water at the same temperature.

Scientific Reason:
Steam produces much more severe burns than boiling water, even though both are at the same temperature of 100°C (373 K).

Explanation:
1. Steam at 100°C possesses extra latent heat energy called the Latent Heat of Vaporization (22.6 × 10⁵ J/kg).
2. When steam comes into contact with skin, it first condenses into boiling water at 100°C, releasing this massive amount of latent heat (2260 kJ/kg) directly onto the skin tissue.
3. It then cools down further as liquid water, releasing additional sensible heat.
4. Boiling water only transfers sensible heat as it cools. Therefore, steam transfers far more energy per gram to the skin, causing deeper and more severe tissue damage.

Processes of Phase Transition:

• A (Solid ⟶ Liquid): Melting / Fusion (Liquefaction upon heating).
• B (Liquid ⟶ Gas): Vaporization / Boiling (Evaporation).
• C (Gas ⟶ Liquid): Condensation (Liquefaction upon cooling).
• D (Liquid ⟶ Solid): Solidification / Freezing.
• E (Solid ⟶ Gas directly): Sublimation (without passing through liquid state).
• F (Gas ⟶ Solid directly): Deposition (Reverse sublimation).