Alloys: mixing metals to make them more useful

1. Pure metals such as copper, gold, iron and aluminium are often too soft for everyday use.
2. An alloy is a mixture of a metal with one or more other elements.
3. Alloys are usually harder than the pure metal, so most metals in everyday use are alloys.

Why alloys are harder than pure metals

1. In a pure metal, the atoms are all the same size and sit in neat layers.
2. These layers can slide over each other easily, which makes the pure metal soft.
3. An alloy contains atoms of different sizes.
   1. The different-sized atoms distort the layers, so they can no longer slide easily, and the alloy is harder.

Everyday alloys: bronze, brass and gold

1. Bronze is an alloy of copper and tin.
2. Brass is an alloy of copper and zinc.
3. Gold used in jewellery is an alloy of gold with metals such as silver, copper and zinc.
   1. The proportion of gold is measured in carats, where 24 carat is pure gold and 18 carat is $\frac{18}{24}=75\%$ gold.

Steels are alloys of iron with carbon

1. Steel is an alloy of iron with carbon and sometimes other metals.
2. High-carbon steel is very hard and strong, but it is brittle.
3. Low-carbon steel is softer and more easily shaped or bent.
4. Stainless steel contains chromium and nickel, making it hard and resistant to corrosion.

Aluminium alloys and choosing the right alloy

1. Aluminium alloys have a low density, so they are strong for their weight.
2. This makes them useful for aircraft, where a low mass is important.
3. An alloy is chosen by matching its properties to the job: hardness, density, resistance to corrosion and cost.
4. No single alloy is best for everything, so the choice always depends on what the object must do.