When quicklime dissolves in water?

Analyze the question: Quicklime is calcium oxide (CaO). The question asks what happens when it dissolves (reacts) in water.

Concept: CaO is a basic oxide. It reacts with water in a strongly exothermic reaction to form calcium hydroxide (slaked lime):

CaO(s) + H₂O(l) → Ca(OH)₂(aq/s) + heat

This is called slaking of lime. So much heat is released that the water can hiss, steam, and the mixture becomes very hot.

Step by step:

• Option A: Heat absorbed would mean an endothermic reaction, with the mixture getting cold. This is wrong, because the mixture gets hot.
• Option B: Heat is evolved. This is correct, as the reaction is exothermic.
• Option C: Oxygen is liberated. This is wrong, because no oxygen gas forms. The oxide's oxygen ends up in the hydroxide.
• Option D: Hydrogen is given off. This is wrong. Hydrogen is released when reactive metals such as Na, K, or Ca (the metal, not its oxide) react with water. CaO is already oxidized, so no H₂ forms.

Answer: B. Heat is evolved.

Exam tip: Remember "quicklime + water = hot." Reactions of the oxides of Group 1 and 2 metals with water are generally exothermic and produce hydroxides, not gases. Gas (H₂) comes off only when the pure metal reacts with water.