UPSC CDS 2 2023 GK Paper

Concept:Slaked lime ($\mathrm{Ca(OH)_2}$) reacts with atmospheric carbon dioxide ($\mathrm{CO_2}$) to form a protective layer of calcium carbonate ($\mathrm{CaCO_3}$) on walls during whitewashing.Explanation:Whitewashing uses slaked lime, $\mathrm{Ca(OH)_2}$, which is prepared by adding water to quicklime ($\mathrm{CaO}$).The reaction occurs slowly with $\mathrm{CO_2}$ present in air to give a thin, shiny layer of $\mathrm{CaCO_3}$.Only option D correctly represents the reactants and products with proper states: $\mathrm{Ca(OH)_2}$ is used as an aqueous solution (aq), $\mathrm{CO_2}$ is a gas (g), $\mathrm{CaCO_3}$ is a solid (s), and water is liquid (l).Option A and B incorrectly use $\mathrm{CaO}$ (quicklime) instead of slaked lime, and option C uses $\mathrm{CO_2}$ in liquid form, which is not correct for this atmospheric reaction.Thus, the balanced equation is: $\mathrm{Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)}$.Answer:Option D: $\mathrm{Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)}$