BECE Science 2013 Paper

Concept:Hard water is softened by removing dissolved calcium and magnesium ions.Explanation:Sodium carbonate ($\mathrm{Na_2CO_3}$) reacts with calcium and magnesium salts in hard water to form insoluble precipitates, so the water becomes soft.For example, $\mathrm{Ca^{2+}(aq) + CO_3^{2-}(aq) \rightarrow CaCO_3(s)}$.Alum only coagulates dirt, chlorination kills germs, and filtration removes suspended particles; none of these removes hardness.Answer:B. Addition of sodium carbonate