Describe what is observed when excess sodium hydroxide solution is added to aluminium oxide, and explain why this observation shows that aluminium oxide is an amphoteric oxide.

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

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

Model answer (4 marks)

Aluminium oxide dissolves in the excess sodium hydroxide solution.

The dissolution occurs because Al₂O₃ reacts with the alkali, forming soluble aluminate ions:
Al₂O₃ + 2 NaOH → 2 NaAlO₂ + H₂O. This shows that Al₂O₃ behaves as an acid.

Al₂O₃ also reacts with acids such as HCl or H₂SO₄, producing aluminium salts and water, demonstrating basic behaviour.

Since it reacts with both acids and bases, aluminium oxide is classified as an amphoteric oxide.

Examiner tips

  • Use the word ‘dissolves’ to describe the observation; include the reaction with NaOH to show acidic behaviour; mention the reaction with an acid to show basic behaviour; state amphoteric at the end.
  • Show the balanced equation for the NaOH reaction to demonstrate understanding of the mechanism.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Writing that Al₂O₃ reacts only with acids and ignoring the NaOH reaction; or vice‑versa.
  • Using the wrong formula for the aluminate ion (e.g., NaAlO instead of NaAlO₂).
  • Failing to mention that the dissolution is due to the formation of soluble aluminate ions.

Mark scheme (4 marks)

  1. Aluminium oxide dissolves (in the sodium hydroxide solution)
  2. Aluminium oxide reacts with (or is neutralised by) the alkali / sodium hydroxide, showing it behaves as an acid
  3. Aluminium oxide also reacts with acids (e.g. hydrochloric acid / sulfuric acid)
  4. Because it reacts with both acids and bases/alkalis, it is described as amphoteric

Key terms in this question

amphoteric oxide

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