UPSC NDA I 2021 General Ability Solved Paper

Concept:Common salt ($\mathrm{NaCl}$) is not used in making Plaster of Paris, which is derived from gypsum.Explanation:Plaster of Paris is produced by heating gypsum ($\mathrm{CaSO}_{4} \cdot 2\mathrm{H}_{2}\mathrm{O}$) at $120^\circ \mathrm{C}$.The reaction is: $2(\mathrm{CaSO}_{4} \cdot 2\mathrm{H}_{2}\mathrm{O}) \rightarrow 2(\mathrm{CaSO}_{4}) \cdot \mathrm{H}_{2}\mathrm{O} + 2\mathrm{H}_{2}\mathrm{O}$.No $\mathrm{NaCl}$ is involved in this process.In contrast, $\mathrm{NaCl}$ is a raw material for the other three compounds:• Bleaching powder: made from limestone and chlorine gas (chlorine is produced using $\mathrm{NaCl}$ and $\mathrm{HCl}$).• Baking soda: produced via $\mathrm{NaCl} + \mathrm{H}_{2}\mathrm{O} + \mathrm{CO}_{2} + \mathrm{NH}_{3} \rightarrow \mathrm{NH}_{4}\mathrm{Cl} + \mathrm{NaHCO}_{3}$.• Washing soda: made by the Solvay process using $\mathrm{NaCl}$ and $\mathrm{CaCO}_{3}$: $2\mathrm{NaCl} + \mathrm{CaCO}_{3} \rightarrow \mathrm{Na}_{2}\mathrm{CO}_{3} + \mathrm{CaCl}_{2}$.Hence, Plaster of Paris is the only one that does not use common salt as a raw material.Answer:Plaster of Paris (Option C).