Metals used in engineering are often alloys rather than pure metals. Explain why alloys are harder than pure metals, and describe one use of alloys that makes use of this property.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Steel is an alloy of iron and carbon. It is used extensively in construction and manufacturing because it is significantly harder than pure iron.
Model answer (4 marks)
In a pure metal the atoms are all the same size and are arranged in regular layers that can slide over each other easily. In an alloy atoms of different sizes are present, which distort the regular arrangement of the layers. This distortion makes it more difficult for the layers to slide over each other, so the alloy is harder and stronger than the pure metal.
One use of an alloy that relies on this greater hardness is steel in construction – for example steel beams in bridges and buildings, where the increased hardness and strength give the structure the required durability and load‑bearing capacity.
One use of an alloy that relies on this greater hardness is steel in construction – for example steel beams in bridges and buildings, where the increased hardness and strength give the structure the required durability and load‑bearing capacity.
Examiner tips
- Use the term "layers" and "slide" to show understanding of slip planes.
- Explain the size mismatch of atoms in alloys.
- Give a specific engineering example such as steel in bridges.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing hardness with electrical conductivity.
- Mentioning only the composition of the alloy without explaining the size mismatch effect.
- Giving an example that does not rely on hardness, e.g. colour or magnetism.
Mark scheme (4 marks)
- In a pure metal, the atoms are all the same size and arranged in regular layers that can slide over each other easily.
- In an alloy, atoms of different sizes are present, which distort the regular arrangement / layers.
- This makes it more difficult for the layers to slide over each other, so the alloy is harder / stronger than the pure metal.
- A suitable use of an alloy that relies on greater hardness or strength, e.g. steel used in construction/bridges/buildings, bronze used for tools or medals, duralumin used in aircraft frames.
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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