1 markMCQSection A

Calcium oxide reacts vigorously with water to produce slaked lime.

$\mathrm{CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)}$

This reaction can be classified as:

(A) Combination reaction

(B) Exothermic reaction

(C) Endothermic reaction

(D) Oxidation reaction

Which of the following is a correct option?

Chemical Reactions and Equations

Double Displacement Reaction

Options

(0)(A) and (C)

(1)(C) and (D)

(2)(A), (C) and (D)

(3)(A) and (B)

Official Answer

The correct option is (D).

double displacementexchange of ionsprecipitatecopper sulphidesulphuric acid

Marking Scheme

11 mark for selecting the correct option (D).

Hint

Look at how the ions of the two reacting compounds exchange partners to form two new compounds, including a precipitate.

Quick Oral Answer

When hydrogen sulphide is passed through copper sulphate solution, a double displacement reaction occurs. The copper ions and hydrogen ions exchange their respective anions to form a black precipitate of copper sulphide and sulphuric acid.

Analysis & Explanation

The chemical equation for the reaction is:

$\mathrm{CuSO_4(aq) + H_2S(g) \rightarrow CuS(s) + H_2SO_4(aq)}$

Why the correct option is right: In this reaction, the copper ions ( $\mathrm{Cu^{2+}}$ ) from copper sulphate and hydrogen ions ( $\mathrm{H^+}$ ) from hydrogen sulphide exchange their partners to form copper sulphide ( $\mathrm{CuS}$ ) and sulphuric acid ( $\mathrm{H_2SO_4}$ ). A reaction in which there is an exchange of ions between the reactants to form new compounds is a double displacement reaction.
Why the distractors are wrong:
Combination reaction: This is incorrect because two compounds react to form two new compounds, rather than combining to form a single product.
Displacement reaction: This is incorrect because a single element does not displace another element from its compound; instead, two compounds exchange ions.
Decomposition reaction: This is incorrect because a single reactant does not break down into simpler substances.

Common Mistakes

1Students might confuse this with a simple displacement reaction because of the presence of copper, but here both reactants are compounds exchanging ions.
2Confusing double displacement with combination or decomposition.

Interesting Facts

Copper sulphide ( $\mathrm{CuS}$ ) occurs in nature as a dark indigo-blue mineral called covellite, which is a moderate conductor of electricity.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 1 mark in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Double Displacement Reaction" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2020 paper.