Oxides can be classified as acidic, basic, amphoteric or neutral. State the classification of each of the following oxides and, for each one, give a chemical reason to support your answer: sulfur dioxide (SO₂), calcium oxide (CaO), and carbon monoxide (CO).

Cambridge International IGCSE Chemistry (0620) — 7.2 Oxides · State · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Sulphur dioxide (SO₂) – acidic oxide. It reacts with water to give sulphurous acid, or with bases to give sulphite salts and water.
Calcium oxide (CaO) – basic oxide. It reacts with water to give calcium hydroxide, a strong base.
Carbon monoxide (CO) – neutral oxide. It does not react with acids or bases and does not form a salt.

Examiner tips

  • Use the word ‘acidic’, ‘basic’, ‘neutral’ exactly. Give a single, clear chemical reason for each. Keep each answer to one sentence to fit the 4‑mark structure.

Common mistakes

  • Writing ‘amphoteric’ for CO. Giving a reason that involves a reaction with water for CO. Using ‘sulphuric acid’ instead of the correct acid for SO₂.

Mark scheme (4 marks)

  1. Sulfur dioxide is an acidic oxide
  2. Sulfur dioxide reacts with water to form an acid / reacts with bases or alkalis to form a salt and water (any one valid chemical reason)
  3. Calcium oxide is a basic oxide
  4. Carbon monoxide is a neutral oxide / does not react with acids or bases / does not form a salt

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