Previous Year Questions: Chemical Reactions and Equations - Class 10 PDF Download

Ans: (d) 1:8

• Electrolysis of water decomposes H₂O into hydrogen (H₂) and oxygen (O₂) gases.
• The balanced equation is: 2H₂O → 2H₂ + O₂.
• Molar mass of H₂ = 2 g/mol, O₂ = 32 g/mol.
• From the equation, 2 moles of H₂ (4 g) and 1 mole of O₂ (32 g) are produced.
• Mass ratio of H₂:O₂ = 4:32 = 1:8.

Ans: (a) Salt and water is formed

• (i) HCl + NaOH → NaCl + H₂O (neutralization, forms salt and water).
• (ii) MgO + 2HCl → MgCl₂ + H₂O (forms salt and water).
• (iii) CO₂ + 2NaOH → Na₂CO₃ + H₂O (forms salt and water).
• In all cases, a salt and water are produced, though the salts may vary in nature (neutral or basic).

Ans: (d) Melting of glaciers

• (a) Milk sours due to bacterial action, a chemical change.
• (b) Fermentation of grapes produces alcohol, a chemical reaction.
• (c) Iron nail rusts in humid air, forming Fe₂O₃·nH₂O, a chemical reaction.
• (d) Melting of glaciers is a physical change (solid to liquid, no new substance formed).

Ans: (d) CH₄ + 2O₂ → CO₂ + 2H₂O + Energy

• Natural gas (methane, CH₄) burns in air to produce carbon dioxide and water, releasing energy (exothermic).
• Balanced equation: CH₄ + 2O₂ → CO₂ + 2H₂O.
• Since it’s exothermic, energy is included in the products.
• Option (c) is balanced but omits energy, making (d) correct.

Ans: (c) 2, 3, 1, 3

• The reaction is: 2Al + 3H₂O → Al₂O₃ + 3H₂.
• Balance Al: 2Al on left, 2Al in Al₂O₃ on right.
• Balance O: 3O in 3H₂O on left, 3O in Al₂O₃ on right.
• Balance H: 6H in 3H₂O on left, 6H in 3H₂ on right.

Thus, p=2, q=3, r=1, s=3.

Ans: (c) (i) and (ii)

• (i) Magnesium burns with a dazzling white flame, correct.
• (ii) Forms magnesium oxide (MgO), a white powder, correct.
• (iii) Magnesium does not vaporize significantly; it reacts with oxygen.
• (iv) MgO in water forms Mg(OH)₂, which is basic and turns red litmus blue, not blue litmus red.

Thus, (i) and (ii) are correct.

Ans: (d) 2, 2, 4, 1

• The balanced equation is: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.
• Balance Pb: 2Pb on left, 2Pb in 2PbO on right.
• Balance N: 4N in 2Pb(NO₃)₂, 4N in 4NO₂.
• Balance O: 12O on left (2×6), 12O on right (2×1 in PbO + 4×2 in NO₂ + 2×1 in O₂).

Thus, a=2, b=2, c=4, d=1.

Ans: (d) (ii) and (iv)

• Thermal decomposition requires heat to break down a compound.
• (i) 2AgCl decomposes by light (photolytic), not heat.
• (ii) CaCO₃ → CaO + CO₂ is thermal decomposition.
• (iii) 2H₂O → 2H₂ + O₂ is electrolytic decomposition.
• (iv) 2KClO₃ → 2KCl + 3O₂ is thermal decomposition.

Thus, (ii) and (iv) are correct.

Ans: (b) Both A and R are true, but R is not the correct explanation of A

• A: Decomposition reactions usually require energy (heat, light, or electricity), making them endothermic.
• R: Composting involves microbial breakdown, releasing heat (exothermic).

Both are true, but R describes an exception (biological process), not a general explanation for A.

Ans: (a) Both A and R are true and R is the correct explanation of A

• A: Silver chloride (AgCl) turns grey in sunlight.
• R: It undergoes photolytic decomposition: 2AgCl → 2Ag + Cl₂. Silver (Ag) is grey, causing the color change.

R correctly explains A.

Ans: (d) A is false, but R is true

• A: Exothermic reactions release heat, but not always light (e.g., neutralization reactions).
• R: Combination reactions can be exothermic (e.g., Mg + O₂ → MgO) or endothermic in rare cases.

Thus, A is false, R is true.

Ans:

1. Formation of a new substance (e.g., precipitate or gas).
2. Change in color or temperature (e.g., heat evolved or absorbed).

Explanation:

• Chemical changes produce new substances with different properties.
• Examples: Gas evolution (bubbles), color change, or temperature change indicate a chemical reaction.
• Physical changes (e.g., shape change) do not produce new substances.

Ans: (a) Lead nitrate decomposes to form lead oxide, nitrogen dioxide, and oxygen.
Equation: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
(b) Natural gas (methane) burns to form carbon dioxide, water, and energy.
Equation: CH₄ + 2O₂ → CO₂ + 2H₂O + Energy

• (a) Thermal decomposition of Pb(NO₃)₂ produces a yellow solid (PbO), brown gas (NO₂), and oxygen.
• (b) Combustion of CH₄ is exothermic, producing CO₂, water, and heat/light.

Ans:
(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) NaCl + AgNO₃ → AgCl + NaNO₃

• (a) Neutralization reaction: Balance H (4 on left, 4 on right), N (2 on left, 2 on right), Ca, and O.
• (b) Double displacement: NaCl and AgNO₃ exchange ions, forming insoluble AgCl (precipitate).

Ans:
(a) 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
(b) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
(a) Photosynthesis: Balance C (6), H (24), O (36) on both sides.
(b) Thermal decomposition: Balance Pb (2), N (4), O (12) on both sides.

Ans:
Observations:

1. Green crystals turn white, then brown.
2. Water droplets condense, and a gas with a pungent smell evolves.

Equation: FeSO₄·7H₂O → FeSO₄ + 7H₂O; 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
Explanation:

• Hydrated FeSO₄·7H₂O loses water, forming anhydrous FeSO₄ (white).
• Further heating decomposes FeSO₄ into Fe₂O₃ (brown), SO₂, and SO₃ (pungent gases).

Ans: Equation: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
Types: Double displacement, Precipitation
Explanation:

• Ions exchange to form insoluble BaSO₄ (precipitate) and NaCl.
• Double displacement: Cations and anions swap partners.
• Precipitation: BaSO₄ is insoluble, forming a solid.

Ans:
(a) 2Cu + O₂ → 2CuO
(b) Heat CuO with hydrogen gas: CuO + H₂ → Cu + H₂O

Explanation:
(a) Copper reacts with oxygen to form black copper(II) oxide (CuO).
(b) Reduction with H₂ removes oxygen, regenerating copper metal.

Ans: (a)
(i) Definition: A reaction where a single compound breaks down into two or more simpler substances, usually with energy input (heat, light, or electricity).
Examples:

• Heat: CaCO₃ → CaO + CO₂
• Light: 2AgCl → 2Ag + Cl₂
• Electricity: 2H₂O → 2H₂ + O₂

(ii) Composting: Microbes break down organic matter, releasing energy as heat, making it exothermic.

Explanation:

• Decomposition requires energy to break bonds, except in biological processes like composting.
• Composting involves oxidation of organic matter, releasing energy.

Ans:

• Double Displacement: Two compounds exchange ions to form two new compounds.
  Equation: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
• Displacement: An element displaces another from a compound.
  Equation: Zn + CuSO₄ → ZnSO₄ + Cu

Explanation:

• Double displacement involves ion exchange (e.g., Na⁺ with Ba²⁺).
• Displacement involves an element replacing another (e.g., Zn displaces Cu).

Ans:

• Heating Copper Powder:
  Observation: Copper turns black.
  Phenomenon: Oxidation
  Equation: 2Cu + O₂ → 2CuO
• Copper in Moist Air:
  Observation: Loses shine, forms a green coating.
  Coating: Copper carbonate (Cu₂(OH)₂CO₃), green.

Explanation:

• Heating forms black CuO due to reaction with oxygen.
• In moist air, copper reacts with CO₂, O₂, and H₂O, forming a green patina.

Ans: (a)

• Reason: To ensure the same number of atoms of each element on both sides, maintaining mass conservation.
• Law: Law of Conservation of Mass – Mass of reactants equals mass of products.
• Balanced Equation: 3Zn + 2H₃PO₄ → Zn₃(PO₄)₂ + 3H₂

Explanation:

• Balance Zn: 3Zn → Zn₃(PO₄)₂.
• Balance P: 2H₃PO₄ → 2PO₄.
• Balance H: 6H in 2H₃PO₄ → 6H in 3H₂.
  
  

Ans:
(a) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
(b) CH₄ + 2O₂ → CO₂ + 2H₂O
(c) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

• (a) Iron reacts with steam to form magnetite and hydrogen.
• (b) Methane combustion forms CO₂ and water.
• (c) Cellular respiration oxidizes glucose, producing CO₂ and water.

Ans:
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

• Respiration breaks down glucose with oxygen, releasing energy (heat and ATP).
• The energy release makes it exothermic.
• Occurs in cells to provide energy for biological processes.