Explain why alloys are often used in construction and engineering instead of pure metals.

OCR GCSE Chemistry A: Gateway Science (J248) — C6.1 Improving processes and products · Explain · 4 marks · View as Markdown

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

Steel is an alloy of iron and carbon. It is widely used in the construction of bridges and buildings, whereas pure iron is rarely used for these purposes.

Model answer (4 marks)

An alloy is a mixture of two or more metals (or a metal with another element such as carbon). The different‑sized atoms disturb the regular arrangement of atoms in the metal lattice. This distortion makes it harder for layers of atoms to slide over one another, so the material has greater strength. The alloy also often has better resistance to corrosion than the pure metal.

Examiner tips

Use the exact terms: alloy, mixture, regular arrangement, layers slide, greater strength, resistance to corrosion.
Show the cause–effect chain: different-sized atoms → lattice distortion → reduced slip → higher strength.
Mention both strength and corrosion resistance as reasons for use.

Common mistakes

Confusing alloy with pure metal, or saying alloy is weaker.
Failing to explain the lattice distortion mechanism.
Omitting the corrosion resistance benefit.

Mark scheme (4 marks)

An alloy is a mixture/combination of two or more metals (or a metal with another element such as carbon)
The different-sized atoms disrupt/distort the regular arrangement of atoms in the metal
This makes it harder for layers to slide over each other
This gives the alloy greater strength (and/or greater resistance to corrosion) compared to the pure metal

Key terms in this question

alloy