Iron is an element. Steel is an alloy made mostly from iron, with small amounts of carbon and other metals added. Explain why steel is described as a mixture rather than a compound, and describe one reason why steel is used in preference to pure iron in construction.

OCR A-Level Chemistry A (H432) — 2.1 Atoms, compounds, molecules and equations · Explain · 5 marks · View as Markdown

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

Iron on its own is relatively soft and prone to corrosion. Steel is produced by combining iron with carbon and other metals to improve its properties.

Model answer (5 marks)

Steel is a mixture because it contains at least two different elements – iron and carbon (and sometimes other metals) – that are not chemically bonded together. In a compound the atoms are bonded in a fixed ratio, but in steel the components can be present in variable proportions. Therefore steel is not a compound.

Steel is preferred to pure iron in construction because it has a much greater strength (and also better resistance to corrosion).

Examiner tips

  • Use the definition of a mixture and contrast it with a compound; mention variable proportions.
  • State the specific property that makes steel useful – e.g. greater strength or corrosion resistance.
  • Keep the answer concise – 5 marks can be earned with two short sentences.
  • Use UK spelling (e.g. ‘steel’).

Common mistakes

  • Confusing alloy with compound and saying steel is a compound.
  • Forgetting to mention that the proportions are variable.
  • Using vague phrases like ‘different elements’ without explaining the lack of chemical bonding.

Mark scheme (5 marks)

  1. A mixture contains at least two different elements or compounds that are not chemically bonded together
  2. In a compound the atoms/elements are chemically bonded together, but in steel they are not
  3. In a mixture the components can be present in variable/different proportions (unlike a compound which has a fixed ratio)
  4. Steel has greater strength than pure iron (so it is used in construction)
  5. Steel has greater resistance to corrosion than pure iron

Key terms in this question

mixture · compound · alloy · element

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