UPSC NDA I 2021 General Ability Solved Paper

Concept:This reaction between quick lime and water is a classic example of an exothermic reaction, where heat is released.Explanation:Quick lime ($\mathrm{CaO}$) reacts vigorously with water ($\mathrm{H_2O}$) to form slaked lime ($\mathrm{Ca(OH)_2}$).During this reaction, a large amount of heat energy is given out.The chemical equation is: $\mathrm{CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(s) + heat}$.This release of heat makes the mixture warm, which is a characteristic of exothermic reactions.In everyday life, this reaction occurs when water is added to dry Portland cement to make concrete.An exothermic reaction is defined as a chemical reaction that releases energy in the form of heat or light.The other options are incorrect because:• A displacement reaction involves one element replacing another in a compound.• An endothermic reaction absorbs heat (the opposite of this reaction).• A decomposition reaction involves a single compound breaking down into simpler substances.Thus, the given reaction clearly fits the definition of an exothermic reaction.Answer:Exothermic reaction (Option D).