Class 10 Science Chapter 1 Important Questions with Answers

Question 1

5 Marks

Consider the following chemical equations:

(i) CaO + H₂O → Ca(OH)₂

(ii) 2NO₂ → 2NO + O₂

(iii) CuSO₄ + Fe → FeSO₄ + Cu

(iv) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

(a) Select the combination reaction from the above reactions. Is this reaction exothermic or endothermic?

(b) Which reaction represents a displacement reaction? State the colour change observed in this reaction.

(c) Identify the reaction in which thermal decomposition occurs.

(d) Write the balanced chemical equation for thermal decomposition of ferrous sulphate.

(e) Write the word equation for the double displacement reaction. Is this reaction a redox reaction? Give reason.

Answer:

(a) The combination reaction is:

CaO + H₂O → Ca(OH)₂

This reaction is exothermic because heat is released during the reaction.

(b) The displacement reaction is:

CuSO₄ + Fe → FeSO₄ + Cu

Iron displaces copper from copper sulphate solution.

Colour change: The blue colour of copper sulphate solution changes to green due to formation of iron sulphate.

(c) Thermal decomposition occurs in:

2NO₂ → 2NO + O₂

In this reaction, nitrogen dioxide decomposes on heating.

(d) Balanced chemical equation for thermal decomposition of ferrous sulphate:

2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

(e) Word equation:

Sodium sulphate + Barium chloride → Barium sulphate + Sodium chloride

This reaction is not a redox reaction because there is no change in oxidation states of elements.

Question 2

2 Marks (MCQ Based)

Balance the following chemical equation:

pMnO₂ + qAl → rMn + sAl₂O₃

The values of p, q, r and s respectively are:

(A) 3, 4, 3, 2
(B) 2, 4, 3, 2
(C) 3, 2, 3, 1
(D) 2, 2, 2, 1

Answer:

3MnO₂ + 4Al → 3Mn + 2Al₂O₃

Therefore, the correct answer is:

Option (A) → 3, 4, 3, 2

Question 3

3 Marks

A colourless gas G is produced when eggshells are treated with solution S. The gas G turns lime water milky.

(a) Identify gas G and solution S.

(b) Write their chemical names and formulae.

(c) Write the chemical reaction involved.

Answer:

(a)

Gas G = Carbon dioxide (CO₂)

Solution S = Hydrochloric acid (HCl)

(b)

Eggshell contains calcium carbonate (CaCO₃).

Chemical names and formulae:

Carbon dioxide → CO₂
Hydrochloric acid → HCl
Calcium carbonate → CaCO₃

(c) Chemical reaction:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Lime water turns milky due to formation of calcium carbonate.

Question 4

3 Marks

Describe one activity each to show that a chemical change has occurred in which:

(a) Change in colour takes place.

(b) Change in temperature takes place.

Answer:

(a) Change in colour:

Take blue copper sulphate solution in a test tube and add an iron nail into it. After some time, the blue colour changes to green.

CuSO₄ + Fe → FeSO₄ + Cu

This colour change indicates a chemical reaction.

(b) Change in temperature:

Add water to quick lime (calcium oxide). Heat is produced and the container becomes hot.

CaO + H₂O → Ca(OH)₂ + Heat

This shows an exothermic chemical reaction.

Question 5

5 Marks

(a) Define decomposition reaction.

(b) How can we say that electrolysis of water and decomposition of silver bromide in sunlight are decomposition reactions?

(c) Mention the type of energy involved in each case.

Answer:

(a) A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances.

(b)

In electrolysis of water, water breaks into hydrogen and oxygen gases.

2H₂O → 2H₂ + O₂

In decomposition of silver bromide:

2AgBr → 2Ag + Br₂

In both reactions, one compound breaks into simpler substances. Hence, both are decomposition reactions.

(c)

Electrolysis of water → Electrical energy
Decomposition of silver bromide → Light energy

Question 6

3 Marks

State why the reaction in which calcium oxide forms calcium hydroxide and the reverse reaction are opposite to each other. Justify with chemical equations.

Also name the type of reaction:

Zn + 2CH₃COOH → (CH₃COO)₂Zn + H₂

Answer:

Calcium oxide reacts with water to form calcium hydroxide:

CaO + H₂O → Ca(OH)₂

This is a combination reaction because two substances combine to form one product.

Reverse reaction:

Ca(OH)₂ → CaO + H₂O

This is a decomposition reaction because one compound breaks into simpler substances.

Therefore, both reactions are opposite to each other.

The reaction:

Zn + 2CH₃COOH → (CH₃COO)₂Zn + H₂

is a displacement reaction because zinc displaces hydrogen from acetic acid.