Amphoteric oxide

Amphoteric oxides are oxides which have properties of both acidic and basic oxides. The property is known as amphoterism. Generally non-metallic oxides are acidic and metallic oxides are basic. Some non-metallic oxides are neutral and some metallic and semi-metallic oxides are amphoteric.

As these oxides can act as either an acid or a base, they can neutralize both acids and bases. For example, aluminium oxide (Al₂O₃) reacts with hydrochloric acid to form aluminium chloride and water. With sodium hydroxide solution it reacts into sodium aluminate (NaAlO₂) and water.

Examples of amphoteric oxides and their reactions

Amphoteric oxides have both acidic and basic properties. The oxides of aluminium and zinc can be used as examples. They react with acids and alkalis to form salts and water. The metals become cations in the former case and oxoanions in the latter.

Examples:

ZnO(s) + 2 HNO₃(aq) → Zn(NO₃)₂(aq) + H₂O(l)

ZnO(s) + 2 NaOH(aq) → Na₂ZnO₂(aq) + H₂O(l)

Al₂O₃(s) + 6 HCl(aq) → 2 AlCl₃(aq) + 3 H₂O(l)

Al₂O₃(s) + 2 NaOH(aq) → 2 NaAlO₂(aq) + H₂O(l)

ZnO, BeO, Al₂O₃, MnO₂, Cr₂O₃, PbO, SnO are all amphoteric.

Zincates (ZnO₂^2-) and aluminates (AlO₂^–) have the ending -ate to show that their ions are compound ions containing oxygen – rather like sulphates, carbonates and nitrates. Note that sodium zincate and sodium aluminate are soluble in water.