Describe what an alloy is and explain why alloys are generally harder than pure metals.

Cambridge International IGCSE Chemistry (0620) — 9.3 Alloys and their properties · Describe 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)

An alloy is a mixture of a metal with one or more other elements (metals or non‑metals). In a pure metal the atoms are all the same size and are arranged in regular layers. In an alloy, atoms of different sizes are present – the added element has a different atomic size. The different‑sized atoms distort or disrupt the regular arrangement so the layers cannot slide over each other as easily, making the alloy harder.

Examiner tips

Use the exact definition of an alloy first, then explain the size‑difference mechanism; link ‘harder’ to ‘layers cannot slide’.
Show the cause–effect chain: different sizes → distortion → reduced slip → increased hardness.
Use UK spelling (e.g. ‘distort’).

Common mistakes

Confusing alloy with compound or mixture of non‑metals; not mentioning the size difference; saying alloys are harder because of higher density rather than atomic arrangement.

Mark scheme (4 marks)

An alloy is a mixture of a metal with one or more other elements (metals or non-metals)
In a pure metal the atoms are all the same size and arranged in regular layers
In an alloy, atoms of different sizes are present / the added element has a different atomic size
The different-sized atoms distort / disrupt the regular arrangement so the layers cannot slide over each other as easily, making the alloy harder

Key terms in this question

alloy · pure metal