7.2 Oxides
- Syllabus
- 0620–2026–2027
- Topic
- 7.2
- Level
- —
Learning objectives
7.2.1Classify oxides as acidic• Classify oxides as acidic, including SO 2 and CO2, or basic, including CuO and CaO, related to metallic and non-metallic character7.2.2Amphoteric oxides as oxides that react• Describe amphoteric oxides as oxides that react with acids and with bases to produce a salt and water7.2.3Classify Al 2O3 and ZnO as amphoteric• Classify Al 2O3 and ZnO as amphoteric oxides
Classify acidic and basic oxides
| Oxide type | Typical element character | Reaction pattern | Required examples |
|---|---|---|---|
| acidic | non-metal oxide | reacts with bases to form salt and water | CO₂, SO₂ |
| basic | metal oxide | reacts with acids to form salt and water | CuO, CaO |
CO₂ neutralises calcium hydroxide; SO₂ reacts with alkalis. CuO + 2HCl → CuCl₂ + H₂O, while CaO neutralises acids and acidic oxides.
Use metallic or non-metallic character as the first prediction, then use reaction evidence with acids and bases to confirm the class.
The metal/non-metal pattern is not universal: some oxides are amphoteric and some non-metal oxides such as CO are neutral. Use the specified examples and reaction evidence.
Describe amphoteric oxides
An amphoteric oxide reacts with both acids and bases to form a salt and water.
| Test | Evidence of amphoteric behaviour |
|---|---|
| add dilute acid | oxide reacts or dissolves; salt and water form |
| add aqueous alkali | oxide reacts or dissolves; a different salt and water form |
Place separate samples of the oxide in a named dilute acid and a named aqueous alkali. Reaction in both tests demonstrates amphoteric behaviour; use fresh oxide and controlled amounts for each test.
Amphoteric does not mean neutral or that the oxide forms a pH 7 solution. It means the oxide can react with both acidic and basic reagents.
Classify aluminium oxide and zinc oxide
Aluminium oxide, Al₂O₃, and zinc oxide, ZnO, are amphoteric oxides.
| Oxide | With acid | With aqueous alkali | Classification |
|---|---|---|---|
| Al₂O₃ | reacts to form an aluminium salt and water | reacts to form an aluminate salt and water | amphoteric |
| ZnO | reacts to form a zinc salt and water | reacts to form a zincate salt and water | amphoteric |
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; ZnO + 2HCl → ZnCl₂ + H₂O. Their additional ability to react with alkali distinguishes them from ordinary basic oxides.
Do not classify Al₂O₃ or ZnO as merely basic because they are metal oxides. Their reactions with both acids and bases make them amphoteric.