What is an alloy?

What is an alloy?

An alloy is made by melting a metal and mixing in other elements, then letting it cool. The result is a mixture, not a compound: the elements aren't chemically bonded in fixed proportions, so the recipe can be adjusted to get the properties you want. The AQA GCSE specification notes that most metals in everyday use are alloys.

Why bother? AQA puts it simply: pure metals are too soft for many uses, so they are mixed with other metals to make alloys, which are harder. Pure iron bends too easily for a bridge, and pure gold scratches and wears away too quickly for jewellery.

Why are alloys harder than pure metals?

AQA describes metals as giant structures of atoms with strong metallic bonding: positive metal ions in a regular pattern, held together by a sea of delocalised electrons. In a pure metal, every atom is the same size, so they stack in neat layers. When a force is applied, whole layers can slide over each other, which is why pure metals can be bent and shaped.

In an alloy, atoms of a different size are mixed in. Larger or smaller atoms distort the layers, so the rows are no longer neat and flat. The bumps act like grit between the layers: it takes much more force to make them slide. So the alloy is harder and stronger than the pure metal.

Try it: this question asks you to explain a property of metals using their structure. Our Chemical Bonding course also has a model of the electron sea.

Examples of alloys

The compositions, and properties such as "high carbon steel is strong but brittle" and "aluminium alloys are low density", are from the AQA specification, which asks you to recall a use for each and to interpret information about other alloys.

Gold purity: working with carats

The purity of gold is measured in carats (karats in the US). AQA states that 24 carat is 100% (pure gold) and 18 carat is 75% gold. So the percentage of gold is the carat number out of 24:

• 9 carat: 9 ÷ 24 × 100 = 37.5% gold.
• 14 carat: 14 ÷ 24 × 100 ≈ 58.3% gold.
• Mass of gold in a 12 g, 18 carat ring: 12 × 18 ÷ 24 = 9 g. The other 3 g is the metals added to harden it.

Lower carat gold contains less gold, so it is cheaper and harder wearing, but less golden in colour.

How alloy properties are chosen

Changing the amount of an added element changes the properties, which is why there are thousands of different steels. Adding more carbon to iron makes steel harder and stronger but more brittle. Adding chromium and nickel makes steel resist rusting. Engineers choose the alloy for the job: a soft, easily pressed steel for car panels, a hard steel for drill bits, a light aluminium alloy for an aircraft.

Common mistakes

• Calling an alloy a compound. It is a mixture; the proportions can vary.
• Saying alloys are stronger because their bonds are stronger. The key idea is distorted layers that can't slide easily.
• Saying alloys have no layers. The layers are still there, just distorted.
• Mixing up brass and bronze. Brass is copper and zinc; bronze is copper and tin.
• Thinking all alloys contain only metals. Steel contains carbon, a non-metal.
• Getting carats backwards. Higher carat means more gold: 24 carat is pure.

Read next

Chemistry · Bonding and structureWhy are metals good conductors of electricity?Why metals are good conductors of electricity and heat: delocalised electrons in metallic bonding, how they carry charge, and why other materials don't.