Amphoteric Oxide
Summary: Amphoteric oxides react with BOTH acids and bases to produce a salt and water. The two key examples at IGCSE are aluminium oxide (Al₂O₃) and zinc oxide (ZnO). Tags: igcse chemistry acids-bases oxides Created: 2026-07-14 Last Updated: 2026-07-23
Definition
An amphoteric oxide is an oxide that can behave as BOTH an acid and a base — it reacts with both acids and alkalis to form a salt and water.
This dual behaviour is because the metal in the oxide has intermediate electronegativity — it can act as both an electron donor (basic) and electron acceptor (acidic).
Aluminium Oxide (Al₂O₃)
Aluminium oxide is the most commonly examined amphoteric oxide. It is found naturally as bauxite and has a giant ionic structure with significant covalent character.
Reaction with Acids (Basic Behaviour)
Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
Al₂O₃(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂O(l)
Al₂O₃(s) + 6HNO₃(aq) → 2Al(NO₃)₃(aq) + 3H₂O(l)
In each case: aluminium salt + water is produced.
Reaction with Alkalis (Acidic Behaviour)
Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq)
With hot concentrated sodium hydroxide, sodium aluminate is formed. The simplified equation at IGCSE is:
Al₂O₃(s) + 2NaOH(aq) → 2NaAlO₂(aq) + H₂O(l)
Zinc Oxide (ZnO)
Zinc oxide behaves similarly to aluminium oxide.
Reaction with Acids
ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)
ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l)
Reaction with Alkalis
ZnO(s) + 2NaOH(aq) + H₂O(l) → Na₂Zn(OH)₄(aq)
Simplified for IGCSE: ZnO(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂O(l)
Identification
Amphoteric behaviour is identified by observing that the oxide reacts with both:
- A warm acid (e.g. dilute HCl or H₂SO₄) — the solid dissolves
- A warm alkali (e.g. NaOH solution) — the solid dissolves
If a solid dissolves in acid but NOT in alkali → it is a basic oxide. If a solid dissolves in alkali but NOT in acid → it is an acidic oxide.
Why Is Al₂O₃ Amphoteric?
Aluminium sits on the diagonal borderline between metals and non-metals in the Periodic Table. Its oxide has:
- Ionic character (allowing reaction with acids)
- Covalent character (allowing reaction with alkalis)
This intermediate bonding nature is what gives it amphoteric properties.
Key Facts
- Al₂O₃ and ZnO are the ONLY amphoteric oxides you need to know for IGCSE
- Amphoteric oxides react with BOTH acids and alkalis → salt + water
- Al₂O₃ + HCl → AlCl₃ + H₂O (acid reaction)
- Al₂O₃ + NaOH → NaAlO₂ + H₂O (base reaction)
- The aluminium oxide layer on aluminium metal explains why aluminium appears unreactive despite its high position in the reactivity series