UPSC NDA II 2018 General Ability Solved Paper

Concept:Permanent hardness of water is caused by dissolved chlorides and sulphates of calcium and magnesium, and it cannot be removed by simple boiling.Explanation:Permanent hardness is due to $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ salts (chlorides and sulphates).Boiling only removes temporary hardness (bicarbonates) by decomposing them to carbonates.For permanent hardness, boiling does not precipitate $\mathrm{Ca}^{2+}$ or $\mathrm{Mg}^{2+}$ chlorides or sulphates.Instead, boiling evaporates water, leaving the dissolved salts behind, thus hardness persists.Other methods remove permanent hardness:• Washing soda ($\mathrm{Na}_2\mathrm{CO}_3$) precipitates $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ as carbonates.• Calgon’s method uses sodium hexametaphosphate to sequester $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$.• Ion exchange method replaces $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ with $\mathrm{Na}^+$ or $\mathrm{H}^+$ ions.Hence, boiling is ineffective against permanent hardness.Answer:C. Boiling