WAEC Chemistry 2025 Paper

Concept:Adding sodium hydroxide to a salt solution forms insoluble metal hydroxides. Their colours help identify the metal ion.Explanation:Sodium hydroxide supplies hydroxide ions, $\mathrm{OH^-}$, to the solution.If $\mathrm{Fe^{2+}}$ ions are present, a dirty green gelatinous precipitate of iron(II) hydroxide, $\mathrm{Fe(OH)_2}$, is formed.The reaction is:$\mathrm{Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s) \downarrow}$This green precipitate matches the observation given in the question.Copper ions, $\mathrm{Cu^{2+}}$, would give a blue precipitate.Iron(III) ions, $\mathrm{Fe^{3+}}$, would give a reddish-brown precipitate.Ammonium ions, $\mathrm{NH_4^+}$, do not form a coloured metal hydroxide precipitate.Therefore, the ion responsible is $\mathrm{Fe^{2+}}$.Answer:B. $\mathrm{Fe^{2+}}$