Classifying Oxides
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge IGCSE Chemistry (0620)
Acidic and Basic Oxides
Oxides are classified according to their metallic or non-metallic character:
| Oxide type | Element type | Reacts with | Examples |
|---|---|---|---|
| Acidic oxide | non-metal | bases (forms salt + water) | SO₂, CO₂ |
| Basic oxide | metal | acids (forms salt + water) | CuO, CaO |
In general, moving from metals towards non-metals, oxide character changes from basic towards acidic.
Worked Example: Classifying Given Oxides
- Question: classify calcium oxide, carbon dioxide, manganese(IV) oxide, and sulfur dioxide as acidic or basic. [2]
- Answer: calcium oxide - basic (Ca is a metal); carbon dioxide - acidic (C is a non-metal); manganese(IV) oxide - basic (Mn is a metal); sulfur dioxide - acidic (S is a non-metal)
Amphoteric Oxides
An amphoteric oxide reacts with both acids and bases, forming a salt and water in each case. Aluminium oxide (Al₂O₃) and zinc oxide (ZnO) are the two amphoteric oxides named in the syllabus.
- With acid: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
- With alkali: Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq)
Worked Example: Reaction of Aluminium Oxide with Acid
- Question: aluminium oxide reacts with both acids and bases to form salts. (i) State the type of oxide this makes aluminium oxide. (ii) Name and give the formula of the salt formed with dilute hydrochloric acid. [3]
- Answer (i): amphoteric (oxide)
- Answer (ii): Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; the salt is aluminium chloride, AlCl₃
Worked Example: Defining Amphoteric
- Question: which statement about amphoteric oxides is correct? (A) react with acids but not bases (B) react with acids and bases, forming a salt and water (C) react with acids and bases, forming a salt and hydrogen (D) react with bases but not acids
- Answer: B. Amphoteric oxides react with both, always producing a salt and water - never hydrogen.
An amphoteric oxide behaves as a base when reacting with an acid, and as an acid when reacting with a base
Common Mistakes
- ❌ Thinking amphoteric oxides produce hydrogen gas with acids
- ✓ oxides never release hydrogen with acids - only reactive metals do. Amphoteric oxides give salt + water with both acids and bases
- ❌ Assuming all metal oxides behave the same way
- ✓ most metal oxides are simply basic (react only with acids); only Al₂O₃ and ZnO are amphoteric (react with both)
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