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If two or more metals are melted together, on cooling they form a new metal: an alloy. Therefore, alloys are homogeneous mixtures of metals with other metals or non-metals.
For example, steel is an alloy made up of iron and carbon.
Changing the percentage of carbon and other ingredients in steel significantly affects its properties allowing different types of steel to have different uses in modern technology. This means that the steel can be made hard, springy, rustproof etc.
Often creating an alloy makes the metal much harder and/or stronger. For example, mixing silver or copper with gold makes it much more durable (and cheaper!) to wear as jewellery.
In a pure metal, all the atoms are the same size, allowing the layers of atoms to slide over each other easily, leading to malleability and ductility. But in an alloy with different sized atoms this sliding is more difficult – so the alloy is stronger.
Mixing copper with tin, both of which are relatively soft metals, makes bronze – an alloy that is much harder than either of its components. Copper and tin are easily extracted, so ancient people learned to do this and then to mix the metals producing a much more useful material – the Bronze Age had arrived! Iron is harder to extract – so the Iron Age came later.