WAEC Chemistry 2011 Paper

Concept:Amphoteric oxides react with both acids and bases to form salt and water.Explanation:An amphoteric oxide shows dual nature: it acts as a basic oxide with acids and as an acidic oxide with bases.Among the given metallic oxides, only $\mathrm{Al_2O_3}$ behaves in this way.With hydrochloric acid, $\mathrm{Al_2O_3}$ acts as a base:$\mathrm{Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O}$With sodium hydroxide solution, $\mathrm{Al_2O_3}$ acts as an acid:$\mathrm{Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]}$The other oxides, $\mathrm{Fe_2O_3}$, $\mathrm{MgO}$, and $\mathrm{Na_2O}$, dissolve in acids only, so they are basic oxides and not amphoteric.Therefore, the amphoteric oxide is $\mathrm{Al_2O_3}$.Answer:Option A: $\mathrm{Al_2O_3}$