Alloys, ceramics, polymers and composites

Most metals we use are alloys: mixtures that are harder or more useful than the pure metal.

The purity of gold is measured in carats: 24 carat is pure gold, so 18 carat is 18 ÷ 24 = 75% gold.

Steels are alloys of iron with small amounts of carbon and other metals:

• Soda-lime glass is made by heating sand, sodium carbonate and limestone together. It is used for windows and bottles.
• Borosilicate glass is made from sand and boron trioxide. It melts at a higher temperature than soda-lime glass, so it is used for laboratory glassware and ovenware.
• Clay ceramics, such as pottery and bricks, are made by shaping wet clay and then heating it in a furnace. They are hard but brittle.

A polymer's properties depend on its monomers and the conditions used to make it. Low-density and high-density poly(ethene) are both made from ethene, but with different catalysts and conditions:

• LDPE is flexible — used for carrier bags and squeezy bottles.
• HDPE is stronger and more rigid — used for bottles, pipes and buckets.

Thermosoftening polymers are made of separate, tangled chains with weak forces between them. They melt when heated and can be reshaped.

Thermosetting polymers have cross-links (strong bonds) between the chains. They don't melt when heated, so they are used for plugs and saucepan handles.

A composite is made of two materials combined so that the result has better properties than either one alone:

• the reinforcement — fibres or fragments of one material
• the matrix (or binder) — the other material, which surrounds and holds the reinforcement together