Amphoteric Aluminium Oxide

An amphoteric oxide reacts with both acids and bases. Aluminium oxide is the amphoteric oxide in Period 3.With acids, Al₂O₃ acts as a base, forming an aluminium salt and water:Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
Al₂O₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂OWith bases, Al₂O₃ acts as an acid. It dissolves in sodium hydroxide solution to form sodium aluminate, which contains the [Al(OH)₄]⁻ ion:Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄Why is it amphoteric? Its bonding is between the ionic basic oxides and the covalent acidic oxides. It is ionic, but the Al³⁺ ion is small and highly charged, so it has a high charge density and polarises the oxide ions. This gives the bonding some covalent character, so the oxide shows both basic and acidic behaviour.Aluminium oxide is insoluble in water, so it does not change the pH of water. Its amphoteric nature is only seen when it is added to an acid or a base.Worked example: how many moles of NaOH react with 5.10 g of Al₂O₃ (M_r = 102.0)? Moles of Al₂O₃ = 5.10 ÷ 102.0 = 0.0500 mol. The ratio is 1 Al₂O₃ : 2 NaOH, so 0.100 mol of NaOH is needed.