Oxides
Summary: Oxides are compounds of an element with oxygen. They are classified as acidic, basic, amphoteric, or neutral based on their reactions with acids and bases. Classification relates to the metallic or non-metallic character of the element. Tags: igcse chemistry acids-bases oxides Created: 2026-07-14 Last Updated: 2026-07-23
Types of Oxides
There are four main types of oxides, classified by their chemical behaviour:
| Type | Reacts with Acids? | Reacts with Bases? | Examples | Element Type |
|---|---|---|---|---|
| Basic | Yes → salt + water | No | CuO, CaO, MgO, Na₂O | Metals |
| Acidic | No | Yes → salt + water | SO₂, CO₂, NO₂, P₄O₁₀ | Non-metals |
| Amphoteric | Yes → salt + water | Yes → salt + water | Al₂O₃, ZnO | Some metals |
| Neutral | No | No | CO, NO, H₂O | Some non-metals |
Basic Oxides
Basic oxides are oxides of metals. They react with acids to form a salt and water.
Examples:
- CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
- CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l)
- MgO(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + H₂O(l)
Properties:
- Metal oxides (especially Group 1 and 2) are basic
- Soluble basic oxides dissolve in water to form alkaline solutions:
- Na₂O(s) + H₂O(l) → 2NaOH(aq)
- CaO(s) + H₂O(l) → Ca(OH)₂(aq)
- Insoluble basic oxides (CuO, Fe₂O₃) do NOT affect pH when added to water
Acidic Oxides
Acidic oxides are oxides of non-metals. They react with bases/alkalis to form a salt and water.
Examples at IGCSE:
- Sulfur dioxide (SO₂): formed by burning sulfur or roasting sulfide ores
- Carbon dioxide (CO₂): formed by combustion of carbon-containing fuels / respiration
- Nitrogen dioxide (NO₂): formed in car engines and lightning
Reactions with alkalis:
- CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)
- SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)
Reactions with water:
- CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) (carbonic acid — makes rain slightly acidic)
- SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq) (sulfurous acid — causes acid rain)
- NO₂(g) + H₂O(l) → HNO₃(aq) + HNO₂(aq) (contributes to acid rain)
Amphoteric Oxides
See Amphoteric Oxide for full details.
Al₂O₃ (aluminium oxide) and ZnO (zinc oxide) are the two amphoteric oxides you need to know for IGCSE.
- With acid: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
- With base: Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq)
Neutral Oxides
Neutral oxides show neither acidic nor basic properties — they do NOT react with acids or bases.
Examples:
- Carbon monoxide (CO)
- Nitrogen monoxide (NO)
- Water (H₂O)
Pattern: Metallic vs Non-Metallic Character
| Element type | Oxide type | Trend |
|---|---|---|
| Metals (left of PT) | Basic | Basicity increases down a group |
| Metalloids (middle) | Amphoteric | Al₂O₃, ZnO |
| Non-metals (right of PT) | Acidic | Acidity increases across a period |
Across Period 3: Na₂O (basic) → MgO (basic) → Al₂O₃ (amphoteric) → SiO₂ (acidic) → P₄O₁₀ (acidic) → SO₂ (acidic) → Cl₂O₇ (acidic)
Key Facts
- Basic oxides = metal oxides → react with acids only
- Acidic oxides = non-metal oxides → react with bases/alkalis only
- Amphoteric oxides = Al₂O₃ and ZnO → react with BOTH acids and bases
- Neutral oxides = CO, NO, H₂O → react with neither
- Acidic oxides cause acid rain when dissolved in rainwater
- Basic oxides from Group 1 metals dissolve in water to form alkalis