A student notices that a piece of iron left outside in the rain develops a reddish-brown coating. Explain why iron rusts and describe two ways in which rusting can be prevented.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iron is a transition metal widely used in construction and engineering. Rusting is an example of a chemical reaction that causes significant economic damage each year.
Model answer (5 marks)
Rusting is the oxidation of iron:
1. Iron reacts with oxygen and water:
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
2. Fe(OH)₃ dehydrates to form iron(III) oxide‑hydrate (rust).
3. Painting the iron creates a physical barrier that blocks both O₂ and H₂O from reaching the metal surface.
4. Galvanising coats the iron with zinc; zinc preferentially oxidises, protecting the iron.
5. Both methods prevent the necessary reactants from contacting the iron, stopping rust formation.
1. Iron reacts with oxygen and water:
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
2. Fe(OH)₃ dehydrates to form iron(III) oxide‑hydrate (rust).
3. Painting the iron creates a physical barrier that blocks both O₂ and H₂O from reaching the metal surface.
4. Galvanising coats the iron with zinc; zinc preferentially oxidises, protecting the iron.
5. Both methods prevent the necessary reactants from contacting the iron, stopping rust formation.
Examiner tips
- Use the exact reaction equation, mention Fe(III) oxide hydrate; name two distinct methods; explain how each blocks O₂/H₂O or provides sacrificial protection; keep answer concise and use UK spelling.
Common mistakes
- Writing only one prevention method; confusing rust with iron oxide (Fe₂O₃) instead of hydrated form; not explaining how the method blocks oxygen or water.
Mark scheme (5 marks)
- Rusting requires both oxygen (from air) and water to be present
- The product formed is iron(III) oxide / hydrated iron(III) oxide (rust)
- First named prevention method (e.g. painting, oiling/greasing, plastic coating, galvanising, sacrificial anode)
- Correct explanation of how first method prevents rusting — prevents oxygen and/or water reaching the iron surface
- Second named prevention method that is different from the first, with a correct explanation or statement that it creates a barrier / protects iron
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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