Alloys and Their PropertiesCambridge IGCSE Chemistry: Revision notes

Section 1

What Is an Alloy?

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

Common examples:

Brass — copper and zinc
Stainless steel — iron with chromium, nickel and carbon

Alloys are made deliberately because pure metals often don't have the properties needed for practical use.

Key termsalloybrassstainless steel

Section 2

How Do the Physical Properties of Metals Compare with Non-Metals?

Before looking at alloys, it helps to know how metals generally compare with non-metals:

Property	Metals	Non-metals
Thermal conductivity	Good	Poor
Electrical conductivity	Good	Poor (except graphite)
Malleability/ductility	Malleable and ductile	Brittle
Melting/boiling points	Generally high	Generally lower

Alloying changes these baseline metallic properties, usually to make the material harder and stronger.

Key termsmalleableductile

Section 3

Why Are Alloys Harder and Stronger Than Pure Metals?

Alloys can be harder and stronger than the pure metals they are made from, and are more useful as a result.

The explanation lies in structure:

In a pure metal, atoms are all the same size and arranged in uniform layers that can slide over each other relatively easily, making pure metals soft and easily bent.
In an alloy, the added atoms are usually a different size to the original metal atoms.
These different-sized atoms distort the regular layers, preventing the layers from sliding over one another as easily.
This makes the alloy harder and stronger than the pure metal.

Key termsdistortion of layers

Think of it like this

Think of a pure metal as a stack of identical, smooth marbles that can roll past each other easily. Mixing in different-sized marbles jams the stack, so it's much harder for the layers to slide.

Common mistake

Don't say alloys are strong because the atoms 'bond more strongly' — the key idea is the size difference disrupting the sliding of layers, not a change in the type of bonding.

Section 4

Why Are Alloys Used in Practice?

Alloys are chosen for specific uses because of the properties this structural change gives them:

Stainless steel in cutlery — chosen for its hardness and resistance to rusting
Brass — harder than pure copper, used where extra strength and a decorative gold-like appearance are wanted

In general, alloys are used wherever a pure metal alone would be too soft, too weak, or otherwise unsuitable for the job.

Example

Pure iron is relatively soft and rusts easily; alloying it with chromium, nickel and carbon to make stainless steel produces cutlery that is hard-wearing and resistant to corrosion.

Must Know

An alloy is a mixture of a metal with other elements
Brass = copper + zinc; stainless steel = iron + chromium + nickel + carbon
Alloys can be harder and stronger than the pure metals used to make them, and so are more useful
Pure metals have atoms of the same size arranged in layers that slide easily over each other
Adding atoms of a different size distorts these layers, stopping them sliding as easily — this is why alloys are harder
Stainless steel is used for cutlery due to its hardness and resistance to rusting

That's the notes covered.

Carry on to the next subtopic.