NCERT Solutions - Class 10 Science Chapter 1: Chemical Reactions and Equations
In-Text Questions (Page No. 06)
Q1. Why should a magnesium ribbon be cleaned before burning in air?
Answer - Magnesium ribbon should be cleaned before burning to remove the layer of magnesium oxide that forms on its surface due to reaction with air. This ensures that the magnesium metal burns properly and gives accurate results.
Q2. Write the balanced equation 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
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer -
i. H₂(g) + Cl₂(g) → 2HCl(g)
ii. 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
iii. 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
i. H₂(g) + Cl₂(g) → 2HCl(g)
ii. 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
iii. 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
Q3. Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer -
i. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
ii. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
i. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
ii. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
In-Text Questions (Page No. 10)
Q1. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Answer -
i. The substance ‘X’ used for whitewashing is quicklime. Its chemical formula is CaO (calcium oxide).
ii. The reaction of calcium oxide with water is:
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
This reaction produces slaked lime, which is used for whitewashing.
i. The substance ‘X’ used for whitewashing is quicklime. Its chemical formula is CaO (calcium oxide).
ii. The reaction of calcium oxide with water is:
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
This reaction produces slaked lime, which is used for whitewashing.
Q2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer - In Activity 1.7, water is electrolysed and it decomposes into hydrogen and oxygen gases. The ratio of hydrogen to oxygen formed is 2:1. Therefore, the amount of gas collected in one test tube is double because it is hydrogen gas (H₂), which is produced in a larger quantity compared to oxygen (O₂).
In-Text Questions (Page No. 13)
Q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer - When an iron nail is dipped in copper sulphate solution, iron displaces copper from the solution because iron is more reactive than copper. This forms iron sulphate and copper is deposited. As a result, the blue colour of copper sulphate fades.
Reaction: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Reaction: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Q2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer - An example of a double displacement reaction is the reaction between sodium carbonate and calcium chloride:
Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq)
This reaction forms an insoluble salt (calcium carbonate) and is also known as a precipitation reaction.
Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq)
This reaction forms an insoluble salt (calcium carbonate) and is also known as a precipitation reaction.
Q3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Answer -
i. Sodium (Na) is oxidised to sodium oxide (Na₂O), and oxygen (O₂) is reduced.
ii. Hydrogen (H₂) is oxidised to water (H₂O), and copper oxide (CuO) is reduced to copper (Cu).
i. Sodium (Na) is oxidised to sodium oxide (Na₂O), and oxygen (O₂) is reduced.
ii. Hydrogen (H₂) is oxidised to water (H₂O), and copper oxide (CuO) is reduced to copper (Cu).
Exercises
Q1. Which of the 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 oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Answer - Correct option: (i) (a) and (b)
(a) is correct, but (b) is incorrect. CO₂ is not getting oxidised, it is the product formed after carbon is oxidised.
(a) is correct, but (b) is incorrect. CO₂ is not getting oxidised, it is the product formed after carbon is oxidised.
Q2. Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Answer - Correct option: (d) displacement reaction.
This reaction is a displacement reaction because aluminium displaces iron from iron oxide.
This reaction is a displacement reaction because aluminium displaces iron from iron oxide.
Q3. 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.
(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 - Correct option: (a) Hydrogen gas and iron chloride are produced.
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)
Q4. 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 of the equation. It follows the Law of Conservation of Mass, which states that mass can neither be created nor destroyed. Balancing ensures that the total mass and the number of atoms of each element remain the same before and after the reaction.
Q5. 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.
(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) → 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
(b) 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
Q6. 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
(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
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
Q7. 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
(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(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(b) Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(c) 2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
(d) BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)
(b) Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(c) 2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
(d) BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)
Q8. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(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)
(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
(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
Q9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer - Exothermic reactions release heat during the reaction. Example: C(s) + O₂(g) → CO₂(g) + heat
Endothermic reactions absorb heat from the surroundings. Example: CaCO₃(s) → CaO(s) + CO₂(g)
Endothermic reactions absorb heat from the surroundings. Example: CaCO₃(s) → CaO(s) + CO₂(g)
Q10. Why is respiration considered an exothermic reaction? Explain.
Answer - Respiration is considered an exothermic reaction because it releases energy. During respiration, glucose combines with oxygen to form carbon dioxide and water, releasing heat.
Reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer - In decomposition reactions, a compound breaks into two or more substances, while in combination reactions, two or more substances combine to form a compound. Thus, they are opposites.
Decomposition: 2H₂O(l) → 2H₂(g) + O₂(g)
Combination: 2H₂(g) + O₂(g) → 2H₂O(l)
Decomposition: 2H₂O(l) → 2H₂(g) + O₂(g)
Combination: 2H₂(g) + O₂(g) → 2H₂O(l)
Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer - Thermal: CaCO₃(s) → CaO(s) + CO₂(g)
Photolytic: 2AgCl(s) → 2Ag(s) + Cl₂(g) [in sunlight]
Electrolytic: 2H₂O(l) → 2H₂(g) + O₂(g) [using electricity]
Photolytic: 2AgCl(s) → 2Ag(s) + Cl₂(g) [in sunlight]
Electrolytic: 2H₂O(l) → 2H₂(g) + O₂(g) [using electricity]
Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer -
| Displacement Reaction | Double Displacement Reaction |
|---|---|
| One element displaces another from its compound. | Exchange of ions takes place between two compounds. |
| Example: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) | Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) |
Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer - Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Q15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer - A precipitation reaction is a chemical reaction in which two aqueous solutions react to form an insoluble solid (precipitate).
Example: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Example: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Q16. Explain the following in terms of gain or loss of oxygen and hydrogen with two examples each.
(a) Oxidation
(b) Reduction
(a) Oxidation
(b) Reduction
Answer - (a) Oxidation:
Oxidation is the process in which a substance gains oxygen or loses hydrogen.
Examples:
1. When magnesium burns in air, it gains oxygen to form magnesium oxide:
2Mg(s) + O2(g) → 2MgO(s)
2. When hydrogen burns in air, it gains oxygen to form water:
2H2(g) + O2(g) → 2H2O(l)
(b) Reduction:
Reduction is the process in which a substance loses oxygen or gains hydrogen.
Examples:
1. When copper oxide is heated with hydrogen, it loses oxygen and gains hydrogen to form copper and water:
CuO(s) + H2(g) → Cu(s) + H2O(g)
2. When iron oxide reacts with hydrogen, it loses oxygen and gains hydrogen to form iron and water:
Fe2O3(s) + 3H2(g) → 2Fe(s) + 3H2O(g)
Oxidation is the process in which a substance gains oxygen or loses hydrogen.
Examples:
1. When magnesium burns in air, it gains oxygen to form magnesium oxide:
2Mg(s) + O2(g) → 2MgO(s)
2. When hydrogen burns in air, it gains oxygen to form water:
2H2(g) + O2(g) → 2H2O(l)
(b) Reduction:
Reduction is the process in which a substance loses oxygen or gains hydrogen.
Examples:
1. When copper oxide is heated with hydrogen, it loses oxygen and gains hydrogen to form copper and water:
CuO(s) + H2(g) → Cu(s) + H2O(g)
2. When iron oxide reacts with hydrogen, it loses oxygen and gains hydrogen to form iron and water:
Fe2O3(s) + 3H2(g) → 2Fe(s) + 3H2O(g)
Q17. 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. On heating, it forms copper oxide (CuO), which is black in colour.
Reaction: 2Cu(s) + O₂(g) → 2CuO(s)
Reaction: 2Cu(s) + O₂(g) → 2CuO(s)
Q18. Why do we apply paint on iron articles?
Answer - We apply paint on iron articles to prevent rusting. The paint forms a protective layer that prevents moisture and oxygen from coming in contact with iron.
Q19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer - Oil and fat-containing food items are flushed with nitrogen to prevent rancidity. Nitrogen displaces oxygen and thus prevents oxidation of fats and oils.
Q20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
(a) Corrosion
(b) Rancidity
Answer - a. Corrosion: It is the process in which metals are gradually eaten up by the action of air, moisture, or a chemical. Example: Rusting of iron.
b. Rancidity: It is the spoilage of food containing fats and oils due to oxidation. Example: Chips become stale and smell bad when exposed to air for long.
b. Rancidity: It is the spoilage of food containing fats and oils due to oxidation. Example: Chips become stale and smell bad when exposed to air for long.