Stainless steel is an alloy used to make surgical instruments. It contains iron, chromium and nickel. Explain why stainless steel is a more suitable material for surgical instruments than pure iron.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Surgical instruments must be strong, durable and resistant to corrosion. Pure iron corrodes readily when exposed to water and oxygen. Stainless steel is an alloy made from iron, chromium and nickel.
Model answer (4 marks)
Stainless steel is an alloy made by combining iron with chromium and nickel.
The addition of chromium and nickel introduces atoms of different sizes into the iron lattice, which disrupts the regular arrangement of atoms.
Because the layers of atoms cannot slide over each other as easily, the alloy is harder and stronger than pure iron.
Chromium also forms a thin, protective oxide layer on the surface, giving stainless steel a much greater resistance to corrosion by water and oxygen than pure iron.
The addition of chromium and nickel introduces atoms of different sizes into the iron lattice, which disrupts the regular arrangement of atoms.
Because the layers of atoms cannot slide over each other as easily, the alloy is harder and stronger than pure iron.
Chromium also forms a thin, protective oxide layer on the surface, giving stainless steel a much greater resistance to corrosion by water and oxygen than pure iron.
Examiner tips
- Show the alloy composition first, then explain atomic size mismatch, link to hardness/strength, finish with corrosion resistance.
- Use the exact phrases from the mark scheme: ‘alloy’, ‘different sized atoms’, ‘layers cannot slide’, ‘harder/stronger’, ‘greater resistance to corrosion’.
- Keep the answer concise – 4 points, no extra words.
Common mistakes
- Failing to mention that chromium and nickel are added to iron; only saying ‘it’s an alloy’. Using vague terms like ‘better’ or ‘more durable’ without linking to atomic structure or corrosion. Omitting the explanation that chromium forms a protective oxide layer.”
Mark scheme (4 marks)
- Stainless steel is an alloy / made by combining two or more metals together
- The different sized atoms disrupt the regular arrangement / layers of atoms cannot slide over each other as easily
- This makes stainless steel harder / stronger than pure iron
- Stainless steel has greater resistance to corrosion than pure iron / less likely to be destroyed by chemical reactions with water and oxygen
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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