WAEC Chemistry 2022 Paper

Concept:Carbonate ions produce a white precipitate with barium chloride, and the precipitate dissolves in dilute acid with effervescence due to carbon dioxide gas.Explanation:Adding $\mathrm{BaCl_2(aq)}$ to salt $X$ gives a white precipitate, which indicates the formation of an insoluble barium salt.When dilute $\mathrm{HCl}$ is added, the precipitate dissolves with rapid effervescence, showing that a gas is evolved.For carbonate ions, the reactions are:$\mathrm{Ba^{2+} + CO_3^{2-} \rightarrow BaCO_3(s)}$ (white precipitate)$\mathrm{BaCO_3(s) + 2HCl(aq) \rightarrow BaCl_2(aq) + H_2O(l) + CO_2(g)}$The carbon dioxide gas released produces the observed effervescence.Sulphate ions would form insoluble $\mathrm{BaSO_4}$, which does not dissolve in dilute $\mathrm{HCl}$.Nitrate ions do not form such a white precipitate with $\mathrm{BaCl_2}$.Answer:Salt $X$ is likely to contain $\mathrm{CO_3^{2-}}$ ions, so the correct option is D.