SAQ : Explain with a chemical equation how hard water can be softened by

Question

Explain with a chemical equation how hard water can be softened by (i) boiling, (ii) use of sodium carbonate. 

Answer

(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.