JAMB Chemistry 2012 Paper

Concept:Hard water is softened by removing dissolved calcium and magnesium ions using chemicals that form insoluble precipitates.Explanation:Soluble sodium compounds, such as sodium carbonate ($\mathrm{Na_2CO_3}$), are added to hard water.These sodium compounds react with calcium and magnesium ions to form insoluble precipitates, e.g. calcium carbonate ($\mathrm{CaCO_3}$) and magnesium carbonate ($\mathrm{MgCO_3}$).This removes the hardness-causing ions from the water.Answer:C. soluble sodium compounds which form insoluble precipitates of calcium and magnesium ions