Class 10 Science
Chapter - 1: Chemical Reactions and Equations
Textual Questions and Answers:
1. Why should a magnesium ribbon be cleaned before burning in air?
Answer: Magnesium ribbon is cleaned before burning because it quickly forms a white layer of magnesium oxide when exposed to air, which does not burn.
2. Write the balanced equations for the following chemical reactions:
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
(i) H₂ + Cl₂ → 2HCl
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) 2Na + 2H₂O → 2NaOH + H₂
3. Write balanced chemical equations with state symbols for the following reactions:
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and a solution of sodium chloride.
(ii) Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride and water.
Answer:
(i) BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)
Textual Questions and Answers:
1. A solution of substance 'X' is used for white-washing.
(i) Name the substance 'X' and write its formula.
(ii) Write the reaction of substance 'X' with water.
Answer:
(i) 'X' is quicklime; its chemical formula is CaO.
(ii) CaO + H₂O → Ca(OH)₂
2. Why is the amount of gas collected in one test tube during electrolysis of water double that collected in the other? Name the gas.
Answer: In the electrolysis of water, the decomposition reaction produces hydrogen and oxygen in a 2:1 ratio. Hydrogen gas is collected in double the amount compared to oxygen.
Chemical equation:
2H₂O → 2H₂ + O₂
Gas collected in greater quantity: Hydrogen.
Textual Questions and Answers:
1. Why does the colour of copper sulphate solution change when an iron nail is immersed in it?
Answer: Iron displaces copper from copper sulphate solution, forming green-coloured iron sulphate.
Chemical reaction:
CuSO₄ + Fe → FeSO₄ + Cu
2. Give an example of a double displacement reaction.
Answer: Reaction between sodium carbonate and calcium chloride forms calcium carbonate precipitate and sodium chloride.
Na₂CO₃ (aq) + CaCl₂ (aq) → CaCO₃ (s) + 2NaCl (aq)
3. Identify the substances oxidized and reduced in the following reactions:
(i) 4Na (s) + O₂ (g) → 2Na₂O (s)
(ii) CuO (s) + H₂ (g) → Cu (s) + H₂O (l)
Answer:
(i) Oxidized: Na, Reduced: O₂
(ii) Oxidized: H₂, Reduced: CuO
Exercises:
1. Which of the following statements about the reaction below are incorrect?
2PbO (s) + C (s) → 2Pb (s) + CO₂ (g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidized.
(c) Carbon is getting oxidized.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All
Answer: (i) (a) and (b)
2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. The above reaction is an example of:
(a) Combination reaction
(b) Double displacement reaction
(c) Decomposition reaction
(d) Displacement reaction
Answer: (d) Displacement reaction.
3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer: (a) Hydrogen gas and iron chloride are produced.
4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: A balanced chemical equation has equal numbers of atoms of each element on both sides. Chemical equations must be balanced to obey the law of conservation of mass.
Class 10 Science
Chapter - 1: Chemical Reactions and Equations
Exercises:
5. Translate the following statements into chemical equations and then balance them:
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) 3H₂(g) + N₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) → 2SO₂(g) + 2H₂O(l)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) 2K(s) + 2H₂O(l) → 2KOH (aq) + H₂(g)
6. Balance the following chemical equations:
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Answer:
(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
7. Write the balanced chemical equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide → Calcium Carbonate + water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium Chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer:
(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl
8. Write the balanced chemical equation for the following reactions and identify the type of reaction:
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium bromide (s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
Answer:
(a) 2KBr (aq) + BaI₂ (aq) → 2KI(aq) + BaBr₂ (s) (Double displacement reaction)
(b) ZnCO₃ (s) → ZnO (s) + CO₂ (g) (Decomposition reaction)
(c) H₂ (g) + Cl₂ (g) → 2HCl (g) (Combination reaction)
(d) Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g) (Displacement reaction)
9. What do you mean by exothermic and endothermic reactions? Give examples.
Answer:
- Exothermic reactions release heat. Example: C (s) + O₂ (g) → CO₂ (g) + Heat.
- Endothermic reactions absorb heat. Example: CaCO₃ → CaO + CO₂ (Heat).
10. Why is respiration considered an exothermic reaction?
Answer: Respiration breaks down glucose with oxygen, releasing energy.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: In decomposition reactions, a single substance breaks down into two or more products. In combination reactions, two or more substances combine to form a single product.
- Decomposition Reaction: 2HgO → 2Hg + O₂
- Combination Reaction: 2H₂ + O₂ → 2H₂O
12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, or electricity.
Answer:
- Thermal decomposition: CaCO₃ → CaO + CO₂ (Heat)
- Photolytic decomposition: 2AgCl → 2Ag + Cl₂ (Light)
- Electrolytic decomposition: 2H₂O → 2H₂ + O₂ (Electricity)
13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer:
- Displacement Reaction: A more reactive element replaces a less reactive element in a compound.
Example: CuSO₄ (aq) + Zn (s) → ZnSO₄ (aq) + Cu (s) - Double Displacement Reaction: Two compounds exchange ions to form new compounds.
Example: Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)
14. In the refining of silver, recovery of silver from silver nitrate solution involves displacement by copper metal. Write the reaction involved.
Answer:
AgNO₃ (aq) + Cu (s) → Cu(NO₃)₂ (aq) + Ag (s)
15. What do you mean by a precipitation reaction? Explain with examples.
Answer: A precipitation reaction occurs when two solutions react to form an insoluble solid.
Example: Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)
16. Explain oxidation and reduction in terms of oxygen gain or loss with two examples each.
Answer:
- Oxidation: Addition of oxygen to a substance.
Example: 2Mg + O₂ → 2MgO - Reduction: Removal of oxygen from a substance.
Example: CuO + H₂ → Cu + H₂O
17. A shiny brown-coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black-coloured compound formed.
Answer: The element 'X' is copper, and the black compound formed is copper oxide (CuO).
Reaction: 2Cu + O₂ → 2CuO
18. Why do we apply paint on iron articles?
Answer: Paint prevents rusting by blocking air and moisture.
19. Oil and fat-containing food items are flushed with nitrogen. Why?
Answer: Nitrogen prevents oxidation, delaying rancidity.
20. Explain corrosion and rancidity with examples.
Answer:
(a) Corrosion: Slow deterioration of metals (e.g., rusting of iron).
(b) Rancidity: Oxidation of fats and oils (e.g., spoiled butter).
This document contains all rewritten questions and answers in an organized manner for clarity and better understanding.
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