JAMB Chemistry 2015 Paper

Concept:Insoluble hydroxide formation occurs when a metal ion reacts with hydroxide ions from a strong base.Explanation:Adding $\mathrm{NaOH}$ solution provides free $\mathrm{OH^-}$ ions.Among the options, only $\mathrm{AlCl_3}$ contains the $\mathrm{Al^{3+}}$ ion.The $\mathrm{Al^{3+}}$ ion combines with $\mathrm{OH^-}$ ions to form a white gelatinous precipitate of aluminium hydroxide:$$\mathrm{Al^{3+}(aq) + 3OH^-(aq) \rightarrow Al(OH)_3(s)\downarrow}$$The precipitate is visible upon addition of $\mathrm{NaOH}$ solution.$\mathrm{NH_4Cl}$ reacts with $\mathrm{NaOH}$ to produce ammonia gas, not a precipitate.$\mathrm{Na_2CO_3}$ and $\mathrm{CH_3COONa}$ are sodium salts and remain soluble, giving no precipitate.Answer:A: $\mathrm{AlCl_3}$