UPSC NDA I 2017 General Ability Solved Paper

Concept:Temporary hardness in water is caused by the presence of dissolved hydrogen carbonates (bicarbonates) of calcium and magnesium.Explanation:The main culprits are $\mathrm{Ca(HCO_3)_2}$ and $\mathrm{Mg(HCO_3)_2}$ in the water.These hydrogen carbonates decompose when the water is heated (boiled).During boiling, they break down to form insoluble carbonates, carbon dioxide gas, and water.The reactions are:$\mathrm{Ca(HCO_3)_2(aq) \rightarrow CaCO_3(s) + CO_2(g) + H_2O(l)}$$\mathrm{Mg(HCO_3)_2(aq) \rightarrow MgCO_3(s) + CO_2(g) + H_2O(l)}$The solid carbonates precipitate out, removing calcium and magnesium ions from the water.This process reduces hardness temporarily, hence the name "temporary hardness".In contrast, chlorides and sulphates (options C and D) cause permanent hardness, which cannot be removed by boiling.Carbonates (option B) are generally insoluble and do not dissolve in water to cause hardness.Therefore, temporary hardness is specifically due to hydrogen carbonates of calcium and magnesium.Answer:A. Hydrogen carbonates