BPSC 66th Prelims 27 Dec 2020 Paper

Concept:Limewater (calcium hydroxide) reacts with carbon dioxide to form an insoluble white precipitate of calcium carbonate, which gives the milky appearance.Explanation:When carbon dioxide ($CO_2$) is passed through limewater ($Ca(OH)_2$), the reaction is: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$.The white solid $CaCO_3$ (calcium carbonate) makes the solution milky.If excess $CO_2$ is passed, the milkiness disappears because $CaCO_3$ converts to soluble calcium bicarbonate: $CaCO_3 + CO_2 + H_2O \rightarrow Ca(HCO_3)_2$.Other gases like $CO$, $O_2$, $O_3$, or atomic oxygen do not produce this precipitate.Thus, the correct gas that turns limewater milky is carbon dioxide ($CO_2$).Answer:Option B: $CO_2$