Mixed on purpose
Pure gold is too soft for jewellery. Jewellers mix it with another metal.
The rule
An alloy mixes a metal with other metals or a non-metal. It is made by melting them together. An alloy can have better properties than the pure metal.
In a pure metal, rows of equal atoms slide easily. Atoms of different sizes block the sliding. A big atom next to a small atom blocks the slide. Each such place adds to the push needed. So an alloy is usually harder than its main metal. An alloy usually conducts less, and often melts at a lower temperature.
| Alloy | Made from | Why it is used |
|---|---|---|
| Brass | Copper and zinc | Harder than copper |
| Bronze | Copper and tin | Hard and strong |
| Solder | Lead and tin | Low melting point, joins wires |
| Steel | Iron and a little carbon | Hard and strong |
| Stainless steel | Iron, nickel and chromium | Resists rust |
| Amalgam | Mercury with a metal such as silver or tin | Dental fillings |
Worked example
Carat counts the parts of gold in 24 parts of the metal. Pure gold is 24 carat. It is very soft. Gold of 18 carat has 18 parts gold, 6 parts other metal. That is 18 ÷ 24 = 0.75, or 75 percent gold. It is hard enough to keep its shape.
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