Alloys

Why mix metals?

Pure iron is soft, and it is easily bent. It also rusts. Pure gold is also very soft. A pure metal is often not the best material for the job. Long ago, people found that mixing a little of a second metal, or even a non-metal, could change a metal for the better. The Bronze Age is named for the alloy that first gave people tools harder than stone and copper.

An alloy is a homogeneous mixture of a metal with one or more other elements. The other elements may be other metals, or a non-metal such as carbon. The metals are melted together and cooled.

Why does mixing make a metal stronger?

In a pure metal, the atoms are all the same size and lie in neat layers. The layers slide over one another easily, and this is why a pure metal bends. In an alloy, atoms of a different size sit among the atoms of the main metal. They disturb the neat layers, and the layers cannot slide so easily. So the alloy is harder.

What alloys do better

• They are harder and stronger. Iron with a little carbon makes steel, which is much stronger than pure iron. A steel with about $0.5\ \%$ of carbon is hard enough for a railway line.
• They resist corrosion. Stainless steel, made of iron with chromium and nickel, does not rust. The chromium forms a thin skin of oxide that protects the steel.
• They can have a lower melting point. Solder melts at a low temperature, so it can join wires.
• They can have a lower conductivity than a pure metal, which is useful for the wire of a heater.
• They can have a better look. Brass looks like gold, and it does not tarnish quickly.

Common alloys

Carat of gold

Pure gold, which is called 24 carat gold, is too soft for jewellery. It is alloyed with copper or silver. In 22 carat gold, $22$ parts in $24$ are gold.

In the same way, 20 carat gold is $\dfrac{20}{24} \times 100 = 83.3\ \%$ gold. A jeweller who sells 20 carat gold sells an alloy that is one part in six other metals.

You should find that a $12$ carat ornament is exactly half gold, whatever its mass.

An alloy is a mixture

Because an alloy is a mixture, the amounts of the metals are not fixed by a formula. Brass can have $70\ \%$ copper and $30\ \%$ zinc, or $60\ \%$ copper and $40\ \%$ zinc. The properties change a little with the amounts. This is different from a compound, which has a fixed ratio.

Practise with new numbers

Try it in Python

This program finds the mass of each metal in $500\ \text{g}$ of four alloys.

Common mistakes

• Thinking that an alloy is a compound. It is a mixture, and its parts can be in different amounts.
• Thinking that an alloy has only metals. Steel has carbon, which is a non-metal.
• Thinking that 24 carat is the best for jewellery. It is the purest, but it is too soft to keep its shape.
• Mixing up brass and bronze. Brass is copper and zinc. Bronze is copper and tin.

Remember these

• An alloy is a mixture of a metal with other elements, melted together.
• Alloys are usually harder and stronger, and they resist corrosion better.
• Brass: copper and zinc. Bronze: copper and tin. Solder: lead and tin. Stainless steel: iron, chromium and nickel.
• 22 carat gold is $91.7\ \%$ gold.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which two metals make bronze?

2. Why is steel stronger than pure iron?

3. What percentage of $20$ carat gold is gold?

4. Why does stainless steel not rust?