Explain with a chemical equation how hard water can be softened by (i) boiling, (ii) use of sodium carbonate.
(i) Boiling: Temporary hardness can be softened by boiling. The chemical equation is:
Ca(HCO3)2 (aq) → CaCO3 (s) + CO2 (g) + H2O (l)
When water is boiled, the bicarbonate ions (HCO3–) decompose, forming insoluble calcium carbonate (CaCO3) that precipitates out, along with the release of carbon dioxide gas (CO2) and water (H2O).
(ii) Use of Sodium Carbonate: Water hardness, both temporary and permanent, can be softened by adding sodium carbonate (Na2CO3). The chemical equation is:
Ca2+ (aq) + Mg2+ (aq) + Na2CO3 (aq) → CaCO3 (s) + MgCO3 (s) + 2Na+ (aq)
Sodium carbonate reacts with the calcium and magnesium ions, forming insoluble calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) precipitates, which can be removed by filtration, thus softening the water.