Explain why sulfur dioxide is classified as an acidic oxide.

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

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

Sulfur dioxide is released into the atmosphere when fossil fuels containing sulfur impurities are burned.

Model answer (4 marks)

Sulphur dioxide reacts with water to give sulphurous acid (H₂SO₃), showing it behaves as an acid.
Sulphur dioxide also reacts with alkalis or bases to give a salt, e.g. Na₂SO₃, without producing water.
These reactions demonstrate that SO₂ is an acidic oxide.

Examiner tips

Use the word ‘acidic oxide’ and give two reactions – with water and with a base. Show the product of the base reaction is a salt, not water. Mention the acid formed (sulphurous acid).

Common mistakes

Calling SO₂ a basic oxide. Forgetting to mention the salt formed with a base. Using the wrong acid name (e.g. sulphuric acid) instead of sulphurous acid.

Mark scheme (4 marks)

Sulfur dioxide reacts with water (to form an acid)
Forms sulfurous acid / H₂SO₃ (named or formula accepted)
Sulfur dioxide reacts with alkalis / bases
To form a salt (only) / no water produced in the reaction with a base

Key terms in this question

acidic oxide