State and explain two reasons why steel (an alloy of iron) is used to make bridges rather than pure iron.

Cambridge International IGCSE Chemistry (0620) — 9.2 Uses of metals · State 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)

1. Steel is stronger and harder than pure iron.
The addition of carbon (and other alloying elements) disturbs the regular iron lattice, making it harder for the iron atoms to slide past each other, so steel has a higher tensile strength.
2. Steel is more resistant to corrosion than pure iron.
Elements such as chromium in stainless steel form a protective layer that prevents oxygen and water from reacting with the iron, reducing oxidation and rusting.

Examiner tips

  • Use the exact terms ‘stronger’, ‘harder’, ‘corrosion resistance’ and explain the role of alloying elements; keep each point concise and separate the reason and explanation clearly.

Common mistakes

  • Confusing steel with pure iron in the explanation; using vague terms like ‘better’ without specifying strength or corrosion; omitting the role of carbon or chromium in the alloy.

Mark scheme (4 marks)

  1. Steel is stronger / harder than pure iron
  2. Explanation: carbon atoms (or other atoms) disrupt the regular layers of iron atoms, preventing them from sliding over each other
  3. Steel is more resistant to corrosion / rusting than pure iron
  4. Explanation: the presence of other elements (e.g. chromium in stainless steel) prevents oxygen and water from reacting with the iron / reduces oxidation

Key terms in this question

alloy

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