Alloy – IGCSE Chemistry Definition

Alloys are tested through a classic 3-mark explanation question: why are alloys harder than pure metals? The answer requires a clear diagram-style understanding of how atoms of different sizes prevent layers from sliding. This appears on both Paper 2 and Paper 4.

The 0620 definition

An alloy is a mixture of a metal with one or more other elements (usually other metals, but sometimes non-metals like carbon).

An alloy is not a compound — no chemical reaction has occurred. The different atoms are mixed together in the metallic lattice.

Why alloys are harder than pure metals

In a pure metal:

• All atoms are the same size
• They are arranged in regular, closely-packed layers
• The layers can slide over each other when force is applied
• This makes pure metals soft and malleable

In an alloy:

• Atoms of different sizes are mixed in
• The different-sized atoms disrupt the regular arrangement
• The layers cannot slide over each other easily
• This makes the alloy harder and stronger

Common alloys

Alloy	Composition	Key property	Use
Steel	Iron + carbon (0.1–1.5%)	Hard, strong	Construction, vehicles
Stainless steel	Iron + chromium + nickel	Corrosion-resistant	Cutlery, surgical instruments
Brass	Copper + zinc	Hard, golden colour	Musical instruments, fittings
Bronze	Copper + tin	Hard, corrosion-resistant	Statues, bearings

Alloys vs pure metals in use

Pure metals have their uses — copper is used for electrical wiring because it is an excellent conductor and is ductile. But for structural applications where strength matters, alloys are almost always preferred.

Pure aluminium is too soft for aircraft bodies. Duralumin (aluminium + copper + other metals) is much stronger while remaining lightweight.

Worked exam question

Explain why brass is harder than pure copper. [3]

Mark scheme

In pure copper, atoms are the same size / arranged in regular layers [1]; the layers can slide over each other [1]; in brass, zinc atoms are a different size, disrupting the regular arrangement / preventing layers from sliding [1]

Common exam mistakes

• Describing alloys as compounds — they are mixtures. The metals are not chemically bonded to each other in a different way; they share the metallic bonding but in an irregular arrangement.
• Saying “the atoms are bonded together more strongly in an alloy” — the hardness comes from disrupting the regular layers, not from stronger bonds.
• Forgetting to mention that the atoms are different sizes — this is the key point the examiner is looking for.