NCERT Solutions for Class 10 Science

Scientific Reason:

Magnesium is a highly reactive metal. When exposed to atmospheric air, it slowly reacts with atmospheric oxygen to form a thin, stable, and protective inert layer of basic magnesium oxide ($MgO$) and basic magnesium carbonate on its surface:

This protective oxide coating prevents further contact of the underlying magnesium metal with oxygen and inhibits ignition. Therefore, before burning in air, the ribbon must be thoroughly rubbed and cleaned with sandpaper to remove this inert oxide layer so that pure magnesium metal comes into direct contact with atmospheric oxygen, allowing it to ignite smoothly with a dazzling white flame.

Step-by-step Balanced Chemical Equations with Physical State Symbols:

1. (i) Hydrogen + Chlorine $\longrightarrow$ Hydrogen chloride:
   $$\text{H}_2(g) + \text{Cl}_2(g) \longrightarrow 2\text{HCl}(g)$$
2. (ii) Barium chloride + Aluminium sulphate $\longrightarrow$ Barium sulphate + Aluminium chloride:
   $$3\text{BaCl}_2(aq) + \text{Al}_2(\text{SO}_4)_3(aq) \longrightarrow 3\text{BaSO}_4(s)\downarrow + 2\text{AlCl}_3(aq)$$ Note: Barium sulphate forms a dense white precipitate.
3. (iii) Sodium + Water $\longrightarrow$ Sodium hydroxide + Hydrogen:
   $$2\text{Na}(s) + 2\text{H}_2\text{O}(l) \longrightarrow 2\text{NaOH}(aq) + \text{H}_2(g)\uparrow + \text{Heat}$$

Identification and Reaction of Substance 'X':

1. (i) Name and Formula:
   The substance 'X' used for white-washing is Calcium oxide (commonly called Quicklime).
   Chemical Formula: $\text{CaO}$.
2. (ii) Chemical Reaction with Water:
   Quicklime reacts vigorously with liquid water in a highly exothermic reaction to produce slaked lime (calcium hydroxide) with the evolution of a large amount of heat and hissing sound: $$\text{CaO}(s) + \text{H}_2\text{O}(l) \longrightarrow \text{Ca(OH)}_2(aq) + \text{Heat}$$
3. Application Note:
   When slaked lime solution is applied to walls, it slowly reacts with carbon dioxide present in the air to form a thin, shiny layer of calcium carbonate ($\text{CaCO}_3$) over 2 to 3 days: $$\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \longrightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$$

Scientific Explanation:

In the electrolysis of water, water molecules ($\text{H}_2\text{O}$) are decomposed into hydrogen gas and oxygen gas by passing an electric current through acidulated water:

1. Mole Ratio: According to the balanced chemical equation and Avogadro's Law (equal volumes of gases contain equal numbers of molecules at the same temperature and pressure), 2 moles (or 2 volumes) of hydrogen gas are liberated for every 1 mole (or 1 volume) of oxygen gas.
2. Electrode Reactions:
   • At Cathode (negative electrode): Hydrogen ions gain electrons: $2\text{H}^+ + 2e^- \longrightarrow \text{H}_2(g)$ [Volume = 2 units].
   • At Anode (positive electrode): Oxygen gas is collected: $2\text{O}^{2-} \longrightarrow \text{O}_2(g) + 4e^-$ [Volume = 1 unit].
3. Conclusion: The gas collected in double the volume is Hydrogen ($\text{H}_2$) because water contains hydrogen and oxygen atoms chemically combined in a fixed $2 : 1$ atomic ratio by volume.

Observation and Chemical Mechanism:

When an iron nail is placed in blue-coloured copper sulphate solution, a single displacement reaction occurs because Iron ($\text{Fe}$) is more reactive than Copper ($\text{Cu}$) in the electrochemical reactivity series.

Chemical Reaction:

Observable Changes:

1. The characteristic deep blue colour of copper sulphate fades gradually and transforms into a light pale green solution due to the formation of ferrous sulphate ($\text{FeSO}_4$).
2. A shining reddish-brown coating of displaced metallic copper gets deposited on the surface of the iron nail.

Detailed Biochemical Explanation:

Respiration is considered an exothermic reaction because energy is released during the metabolic breakdown of food inside living cells.

Mechanistic Steps:

1. During digestion, carbohydrates consumed in our diet (such as rice, potatoes, bread) are broken down into simpler sugars, predominantly glucose ($\text{C}_6\text{H}_{12}\text{O}_6$).
2. This glucose is absorbed into the bloodstream and delivered to tissue cells where it combines with inhaled oxygen in the presence of cellular enzymes.
3. The glucose undergoes slow, controlled combustion (cellular respiration) to form carbon dioxide, water, and releases metabolic energy in the form of ATP (Adenosine Triphosphate): $$\text{C}_6\text{H}_{12}\text{O}_6(aq) + 6\text{O}_2(aq) \longrightarrow 6\text{CO}_2(aq) + 6\text{H}_2\text{O}(l) + \text{Energy (38 ATP)}$$
4. Since substantial energy is liberated to maintain vital cellular functions and body temperature, cellular respiration is classified as an exothermic process.

1. Fundamental Comparison:

• A Combination reaction is a chemical reaction in which two or more simple reactants (elements or compounds) combine together to form a single, more complex product: $$A + B \longrightarrow AB + \text{Heat (usually exothermic)}$$
• A Decomposition reaction is a chemical reaction in which a single complex reactant breaks down into two or more simpler substances upon the absorption of external energy (heat, light, or electricity): $$AB + \text{Energy} \longrightarrow A + B \text{ (always endothermic)}$$
• Thus, decomposition reactions are exact chemical and thermodynamic opposites of combination reactions.

2. Three Modes of Decomposition Reactions with Balanced Equations:

1. Thermal Decomposition (Energy supplied as Heat $\Delta$):
   Heating green crystals of ferrous sulphate breaks it down into ferric oxide, sulphur dioxide, and sulphur trioxide: $$2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)$$ Opposite combination example: $2\text{Mg}(s) + \text{O}_2(g) \longrightarrow 2\text{MgO}(s)$.
2. Electrolytic Decomposition (Energy supplied as Electricity):
   Passing electric current through molten sodium chloride or acidulated water: $$2\text{H}_2\text{O}(l) \xrightarrow{\text{Electric current}} 2\text{H}_2(g) + \text{O}_2(g)$$ Opposite combination example: $2\text{H}_2(g) + \text{O}_2(g) \longrightarrow 2\text{H}_2\text{O}(l)$.
3. Photolytic Decomposition (Energy supplied as Sunlight $h\nu$):
   White silver chloride turns grey in sunlight due to decomposition into silver and chlorine gas: $$2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)$$ Opposite combination example: $2\text{Ag}(s) + \text{Cl}_2(g) \longrightarrow 2\text{AgCl}(s)$.

Concept of Redox (Reduction-Oxidation) Reactions:

• Oxidation: Gain of oxygen or loss of hydrogen (or loss of electrons).
• Reduction: Loss of oxygen or gain of hydrogen (or gain of electrons).
• Oxidising Agent: The substance that gives oxygen or removes hydrogen (gets reduced).
• Reducing Agent: The substance that removes oxygen or adds hydrogen (gets oxidised).

Analysis of the Reaction:

1. Substance Oxidised: $\text{H}_2$ gains oxygen to form $\text{H}_2\text{O}$. Therefore, Hydrogen ($\text{H}_2$) is oxidised.
2. Substance Reduced: $\text{CuO}$ loses oxygen to form metallic $\text{Cu}$. Therefore, Copper(II) oxide ($\text{CuO}$) is reduced.
3. Oxidising Agent: $\text{CuO}$ provides the oxygen and causes oxidation of $\text{H}_2$. Hence, $\text{CuO}$ is the oxidising agent.
4. Reducing Agent: $\text{H}_2$ abstracts oxygen from $\text{CuO}$. Hence, $\text{H}_2$ is the reducing agent.