When iron rusts, it reacts with oxygen and water to form iron oxide. Explain why rusting is a chemical change and not a physical change.

Eduqas GCSE Chemistry — C4 Chemical changes · Explain · 4 marks · View as Markdown

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

Iron gates left outside over winter develop a reddish-brown coating of iron oxide (rust). Rusting is a slow process that happens when iron is exposed to both oxygen and water.

Model answer (4 marks)

Rusting produces a new substance – iron oxide – which has different chemical composition to iron.
The reaction is irreversible; the iron cannot be recovered by simple physical means.
In a physical change no new substance is formed and the original material can be restored.
The properties of iron oxide (colour, hardness, conductivity) differ from those of iron, confirming a chemical change.

Examiner tips

Use the word ‘new substance’ and ‘irreversible’ – key words in the mark scheme.
Show that iron oxide’s properties differ from iron to justify the change.
Keep the answer concise – 4 short points will earn full marks.

Common mistakes

Confusing rusting with corrosion and not stating it’s a new substance.
Claiming the change is reversible or that iron can be recovered easily.
Using vague terms like ‘different’ without linking to chemical composition or properties.

Mark scheme (4 marks)

A new substance (iron oxide) is formed
The change is irreversible / cannot be easily reversed
In a physical change, no new substance is formed / the original substance can be recovered
The properties of iron oxide are different from those of iron, showing a chemical reaction has occurred

Key terms in this question

chemical change · physical change · iron oxide