Assertion (A) : Reaction of Quicklime with water is an exothermic reaction.

Reason (R) : Quicklime reacts vigorously with water releasing a large amount of heat.

Chemical Reactions and Equations

Exothermic Reactions

Options

(a)Both (A) and (R) are true and (R) is the correct explanation of (A).

(b)Both (A) and (R) are true but (R) is not the correct explanation of (A).

(c)(A) is true but (R) is false.

(d)(A) is false but (R) is true.

Official Answer

Both (A) and (R) are true and (R) is the correct explanation of (A).

QuicklimeSlaked limeExothermic reactionHeat released

CaO

Ca(OH)_2

Marking Scheme

11 Mark for selecting the correct option (a).

Hint

Recall Activity 1.4 where water is added to quicklime in a beaker. What happens to the temperature of the beaker, and what does 'exothermic' mean?

Quick Oral Answer

The reaction of quicklime with water is exothermic because it releases a large amount of heat, making the reaction mixture and container hot.

Analysis & Explanation

Assertion (A) is true because the reaction of quicklime (calcium oxide, $CaO$ ) with water is indeed an exothermic reaction. Reason (R) is also true because quicklime reacts vigorously with water to produce slaked lime ( $Ca(OH)_2$ ) and releases a large amount of heat, as shown in the equation: $CaO(s) + H_2O(l) \longrightarrow Ca(OH)_2(aq) + \text{Heat}$ . Since the release of heat is the defining characteristic of an exothermic reaction, the reason correctly explains why the reaction is classified as exothermic. Therefore, both (A) and (R) are true and (R) is the correct explanation of (A).

Common Mistakes

1Thinking that the reaction is endothermic because it requires water.
2Failing to connect the vigorous release of heat with the definition of an exothermic reaction.

Interesting Facts

The reaction of quicklime with water is so exothermic that it can easily boil the water in the container. This reaction was historically used in self-heating cans and portable heaters.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 1 mark in the CBSE Class 10 Science 2023 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 "Exothermic Reactions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2023 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2023 paper.