UPSC NDA 2 2021 General Ability Solved Paper

Concept:Passing carbon dioxide gas through lime water forms an insoluble white precipitate due to a chemical reaction.Explanation:Lime water is an aqueous solution of calcium hydroxide, $\mathrm{Ca(OH)_2}$. When $\mathrm{CO_2}$ gas is bubbled through it, the two substances react chemically. The product formed is calcium carbonate, $\mathrm{CaCO_3}$, which is a white, insoluble solid. This solid appears as a milky white precipitate suspended in the liquid. The balanced chemical equation is: $$\mathrm{Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 \downarrow (white) + H_2O (l)}$$ The downward arrow indicates the formation of a precipitate. Thus, the colour of the precipitate obtained is white. No other colour (green, blue, or brown) is produced under normal conditions.Answer:C. White