CBSE Class X Science Chapter 1: Chemical Reactions and Equations Questions and Answers

Chapter 1: Chemical Reactions and Equations - Questions & Answers

Section 1.1: In-Text Questions (Page 6)

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Magnesium ribbon is cleaned by rubbing with sandpaper before burning to remove any protective layer or impurities from its surface so that it can react properly with oxygen in the air.

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

The balanced chemical equations are:

• (i) H₂ + Cl₂ → 2HCl
• (ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
• (iii) 2Na + 2H₂O → 2NaOH + H₂

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.

The balanced chemical equations with state symbols are:

• (i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
• (ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Section 1.2: In-Text Questions (Page 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) The substance 'X' is calcium oxide (also known as quick lime). Its chemical formula is CaO.

(ii) Reaction of calcium oxide with water:

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

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.

During the electrolysis of water, water decomposes into hydrogen gas and oxygen gas. Since a molecule of water consists of two parts hydrogen and one part oxygen, hydrogen gas is collected in double the amount of oxygen gas. The gas collected in double the quantity is hydrogen gas.

Section 1.3: In-Text Questions (Page 13)

Q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

When an iron nail is dipped in copper sulphate solution, iron displaces copper from copper sulphate solution because iron is more reactive than copper. This forms iron sulphate (FeSO₄), causing the blue color of the copper sulphate solution to fade and change to green.

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.

An example of a double displacement reaction is the reaction between lead nitrate solution and potassium iodide solution:

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

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)

• Substance oxidised: Sodium (Na) - gains oxygen.
• Substance reduced: Oxygen (O₂).

(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

• Substance oxidised: Hydrogen (H₂) - gains oxygen.
• Substance reduced: Copper(II) oxide (CuO) - loses oxygen.

End-of-Chapter Exercises (Pages 14-16)

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.

Options:
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all

Correct Answer: (i) (a) and (b)

Explanation: Lead oxide (PbO) is losing oxygen and getting reduced to lead (Pb), and carbon (C) is gaining oxygen to get oxidised to carbon dioxide (CO₂). Therefore, statements (a) and (b) are incorrect.

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.

Correct Answer: (d) displacement reaction.

Explanation: Aluminium displaces iron from ferric oxide because aluminium is more reactive than iron.

Q3. What happens when dilute hydrochloric acid is added to iron filings? 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.

Correct Answer: (a) Hydrogen gas and iron chloride are produced.

Explanation: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Q4. What is a balanced chemical equation? Why should chemical equations be balanced?

A balanced chemical equation is one in which the total number of atoms of each element is equal on both the reactant side (LHS) and product side (RHS) of the equation.

Chemical equations must be balanced to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. Thus, the total mass of reactants must equal the total mass of products, requiring the number of atoms of each element to 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) 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)

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) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
• (b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
• (c) NaCl + AgNO₃ → AgCl + NaNO₃ (Already balanced)
• (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) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
• (b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
• (c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
• (d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

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) Equation: 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s)
  Type: Double displacement reaction
• (b) Equation: ZnCO₃(s) → ZnO(s) + CO₂(g)
  Type: Decomposition reaction (Thermal decomposition)
• (c) Equation: H₂(g) + Cl₂(g) → 2HCl(g)
  Type: Combination reaction
• (d) Equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
  Type: Displacement reaction

Q9. What does one mean by exothermic and endothermic reactions? Give examples.

Exothermic Reactions: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Example: Burning of natural gas:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat

Endothermic Reactions: Reactions in which energy is absorbed (in the form of heat, light, or electricity) are known as endothermic reactions.

Example: Thermal decomposition of calcium carbonate:
CaCO₃(s) —Heat→ CaO(s) + CO₂(g)

Q10. Why is respiration considered an exothermic reaction? Explain.

We need energy to stay alive, which comes from the food we eat. During digestion, carbohydrates present in food (like rice, potatoes, bread) break down into glucose. This glucose combines with oxygen in the cells of our body to produce energy along with carbon dioxide and water. Since energy is released during this process, respiration is considered an exothermic reaction.

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

In a combination reaction, two or more substances combine to form a single product. In contrast, in a decomposition reaction, a single substance breaks down into two or more simpler substances. Thus, their processes are exact opposites.

Combination Reaction Equation:
2H₂(g) + O₂(g) → 2H₂O(l)

Decomposition Reaction Equation:
2H₂O(l) —Electricity→ 2H₂(g) + O₂(g)

Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

• Decomposition by Heat (Thermal Decomposition):
  CaCO₃(s) —Heat→ CaO(s) + CO₂(g)
• Decomposition by Light (Photochemical Decomposition):
  2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g)
• Decomposition by Electricity (Electrolytic Decomposition):
  2H₂O(l) —Electricity→ 2H₂(g) + O₂(g)

Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Displacement Reaction: A reaction in which a more reactive element displaces or removes another less reactive element from its compound.

Equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Double Displacement Reaction: A reaction in which there is an exchange of ions between the two reactant compounds to form two new products.

Equation: 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.

The chemical reaction involved in the recovery of silver is:

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

Q15. What do you mean by a precipitation reaction? Explain by giving examples.

Any reaction that produces an insoluble solid substance (called a precipitate) which separates out from the solution is called a precipitation reaction.

Example: When sodium sulphate solution is mixed with barium chloride solution, a white precipitate of barium sulphate is formed:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Q16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction

(a) Oxidation: Oxidation is a process that involves the gain of oxygen by a substance.

• Example 1: 2Cu + O₂ —Heat→ 2CuO (Copper gains oxygen to form copper oxide).
• Example 2: C + O₂ → CO₂ (Carbon gains oxygen to form carbon dioxide).

(b) Reduction: Reduction is a process that involves the loss of oxygen by a substance.

• Example 1: CuO + H₂ —Heat→ Cu + H₂O (Copper oxide loses oxygen to form copper).
• Example 2: ZnO + C → Zn + CO (Zinc oxide loses oxygen to form zinc metal).

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.

The shiny brown coloured element 'X' is Copper (Cu).

The black coloured compound formed is Copper(II) oxide (CuO).

Reaction: 2Cu(s) + O₂(g) —Heat→ 2CuO(s)

Q18. Why do we apply paint on iron articles?

We apply paint on iron articles to prevent them from rusting. The layer of paint prevents iron from coming into direct contact with moisture and oxygen (air) present in the atmosphere, thereby stopping corrosion.

Q19. Oil and fat containing food items are flushed with nitrogen. Why?

Oil and fat containing food items are flushed with nitrogen gas to prevent them from getting oxidised. Nitrogen acts as an unreactive gas that replaces atmospheric oxygen, preventing the food from becoming rancid and maintaining its smell and taste.

Q20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity

(a) Corrosion: When a metal is attacked by substances around it such as moisture, air, or acids, it oxidises and deteriorates. This process is called corrosion.

Example: Rusting of iron (formation of a reddish-brown powder on iron surface) or the green coating on copper.

(b) Rancidity: When fats and oils present in food are oxidised, they undergo a change in their smell and taste, rendering the food spoiled. This phenomenon is known as rancidity.

Example: Butter left exposed at room temperature for a long time develops a foul smell and unpleasant taste.