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

Reason (R): A large amount of heat is evolved on the reaction of quick lime and water.

Also asked in:2026

Chemical Reactions and Equations

Combination Reaction — Quick Lime and Water

Options

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

(B)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).

(C)Assertion (A) is true, but Reason (R) is false.

(D)Assertion (A) is false, but Reason (R) is true.

Official Answer

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

Hint

Recheck Activity 1.4 — touching the beaker after adding water to quick lime.

Analysis & Explanation

From Activity 1.4 in the NCERT chapter: when water is slowly added to calcium oxide (quick lime), the reaction is felt to be hot on touching the beaker. The text explicitly states, 'Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat,' represented as

$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat} \quad (1.13)$

Since heat appears as a product term in the equation itself, this combination reaction is exothermic — both the Assertion and the Reason state this same fact, and the Reason (large amount of heat evolved) is precisely why the reaction is classified as exothermic, making it the correct explanation of the Assertion.

Why the other options are wrong:

(B) is incorrect because R does explain A — the evolution of heat is the defining criterion for calling the reaction exothermic, not an unrelated true fact.

(D) is incorrect because A is true, not false — the chapter confirms the reaction is exothermic.

Common Mistakes

1Marking R as unrelated to A when it is in fact the direct cause/definition.
2Confusing this exothermic combination reaction with an endothermic decomposition reaction.

Interesting Facts

The slaked lime solution formed in this reaction is used for whitewashing walls; it reacts slowly with atmospheric $CO_2$ to form a shiny layer of $CaCO_3$ over 2-3 days.

Marble and the calcium carbonate formed on whitewashed walls share the same chemical formula, $CaCO_3$ .

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2026?

This question carries 1 mark in the CBSE Class 10 Science 2026 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 "Combination Reaction — Quick Lime and Water" from CBSE Class 10 Science.

Has this question appeared in other CBSE Science papers?

Yes, a similar question appeared in the 2026 CBSE Class 10 Science paper.

What type of question is this in the CBSE Class 10 Science 2026 paper?

This is a MCQ question from Section B in the CBSE Class 10 Science 2026 paper.