Class 10 Science: Chemical Reactions and Equations – Answer Key
Section A: Multiple Choice Questions
Q1. Answer: (c)
Equation: Fe + CuSO4 → FeSO4 + Cu
Explanation: Iron is more reactive than copper and displaces it from copper sulphate solution.
Q2. Answer: (c) (i), (ii) and (iv)
Explanation: Iron gains oxygen to form Fe3O4, so it is oxidized. Water loses oxygen to form hydrogen and is reduced. Therefore, water acts as the oxidizing agent.
Q3. Answer: (d) (ii) and (iv)
Explanation: It is both a double displacement reaction and a precipitation reaction because insoluble barium sulphate is formed.
Q4. Answer: (b) 2:1
Explanation: The balanced equation is:
2H2O → 2H2 + O2
Thus, hydrogen and oxygen are produced in a 2:1 ratio by volume.
Section B: Assertion-Reason Questions
Q5. Answer: (a)
Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
Explanation: Sunlight causes silver chloride to decompose into silver and chlorine gas.
Q6. Answer: (a)
Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
Explanation: Balancing chemical equations ensures the law of conservation of mass is followed.
Section C: Short Answer Questions
Q7.
Why is magnesium ribbon cleaned before burning?
Magnesium ribbon develops a protective layer of basic magnesium carbonate [MgCO3·Mg(OH)2] when exposed to moist air. Cleaning removes this layer and allows proper burning.
Compound Formed: Magnesium Oxide (MgO)
Colour: White powder
Q8.
(a) Thermal Decomposition:
2FeSO4 → Fe2O3 + SO2 + SO3
(b) Double Displacement / Precipitation:
CuSO4 + H2S → CuS↓ + H2SO4
Q9.
Element X: Copper (Cu)
Black Compound: Copper(II) Oxide (CuO)
Equation:
2Cu + O2 → 2CuO
Q10.
A bright yellow precipitate of lead iodide (PbI2) is formed.
(a) Type of Reaction:
- Double Displacement Reaction
- Precipitation Reaction
(b) Equation:
Pb(NO3)2 + 2KI → PbI2↓ + 2KNO3
Q11.
Exothermic Reactions:
Reactions that release heat energy to the surroundings.
Example:
CH4 + 2O2 → CO2 + 2H2O + Heat
Endothermic Reactions:
Reactions that absorb heat energy from the surroundings.
Example:
CaCO3 → CaO + CO2
Section D: Long Answer Questions
Q12.
(a) Iron is more reactive than copper. Therefore, it displaces copper from copper sulphate solution.
Observations:
- Blue copper sulphate solution turns light green.
- Reddish-brown copper deposits on the iron nail.
Equation:
Fe + CuSO4 → FeSO4 + Cu
(b) Redox Reaction:
A reaction in which oxidation and reduction occur simultaneously.
Substance Oxidized: HCl
Substance Reduced: MnO2
Oxidizing Agent: MnO2
Reducing Agent: HCl
Q13.
(a)
NaOH + HCl → NaCl + H2O
(b)
Pb(CH3COO)2 + 2HCl → PbCl2↓ + 2CH3COOH
(c)
CaO + H2O → Ca(OH)2 + Heat
Section E: Case-Based Questions
Q14.
(a)
CaCO3 → CaO + CO2
Use: Production of quicklime used in cement manufacturing.
(b)
Silver bromide decomposes in the presence of light. Therefore, it is stored in dark-coloured bottles to prevent decomposition.
Equation:
2AgBr → 2Ag + Br2
(c)
Type of Decomposition: Electrolytic Decomposition (Electrolysis)
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