UPSC CDS 1 2021 GK Paper

Concept:Sodium carbonate ($\mathrm{Na}_2\mathrm{CO}_3$) is used to remove the permanent hardness of water by precipitating calcium and magnesium ions.Explanation:Hardness in water is mainly due to dissolved calcium ($\mathrm{Ca}^{2+}$) and magnesium ($\mathrm{Mg}^{2+}$) salts.Temporary hardness is removed by boiling, while permanent hardness requires chemical treatment.Sodium carbonate (washing soda, $\mathrm{Na}_2\mathrm{CO}_3 \cdot 10\mathrm{H}_2\mathrm{O}$) reacts with these ions.For example: $\mathrm{Ca}^{2+} + \mathrm{CO}_3^{2-} \rightarrow \mathrm{CaCO}_3 \downarrow$ (insoluble precipitate).Similarly, $\mathrm{Mg}^{2+} + \mathrm{CO}_3^{2-} \rightarrow \mathrm{MgCO}_3 \downarrow$ .The precipitates can be filtered out, leaving softened water.Other sodium compounds like $\mathrm{NaHCO}_3$ (baking soda) or $\mathrm{NaOH}$ (caustic soda) are not typically used for large-scale softening.$\mathrm{Na}_2\mathrm{SO}_4$ does not precipitate hardness-causing ions effectively.Answer:The correct option is A: $\mathrm{Na}_2\mathrm{CO}_3$.