UPSC CDS 1 2021 GK Paper

Concept:The soda-acid fire extinguisher uses a reaction that produces carbon dioxide gas to smother flames.Explanation:The main chemical used in a soda-acid fire extinguisher is sodium hydrogen carbonate (sodium bicarbonate).The extinguisher contains a solution of $\mathrm{NaHCO}_3$ in water and a separate bottle of sulfuric acid ($\mathrm{H}_2\mathrm{SO}_4$).When the knob is turned, the acid mixes with the bicarbonate solution.The reaction produces carbon dioxide gas:$2\mathrm{NaHCO}_3 + \mathrm{H}_2\mathrm{SO}_4 \rightarrow\mathrm{Na}_2\mathrm{SO}_4 + 2\mathrm{H}_2\mathrm{O} + 2\mathrm{CO}_2$Carbon dioxide is non-combustible and heavier than air.It forms a layer over the fuel, cutting off the oxygen supply.This simultaneously cools the material below its ignition temperature.Thus, the flame is extinguished without using water on electrical or oil fires.Other options: sodium chloride (table salt) does not produce $\mathrm{CO}_2$; calcium hydroxide (lime) is a base; acetic acid (vinegar) is not used because it is weak and less effective.Answer:Sodium hydrogen carbonate (option B).