The material chosen for a job depends on its properties. Useful properties include strength, hardness, density, melting point, flexibility and whether the material conducts heat or electricity. To choose a material, work out which properties the job needs, then match them to the data. Often one property decides the choice and the others just have to be good enough.
Metals are strong, malleable and good conductors of heat and electricity, so copper is used for electrical wires and steel for bridges. Most metals have high densities. Aluminium has a density of about 2.7 g/cm3 compared with about 7.9 g/cm3 for steel, so aluminium is used where a light metal is needed, such as aircraft bodies. Polymers have low densities, are flexible and are good insulators, so poly(ethene) is used for bags and the plastic coating on wires. Their melting points are low, so they soften when hot.
Glass and clay ceramics are hard and have high melting points, and they are electrical and thermal insulators. They are brittle, so they shatter or crack under a sharp impact. Clay ceramics are used for bricks and for the insulators on electricity pylons, and glass is used for windows because it is transparent.
A composite is made of two or more materials joined together so that it has properties that neither material has alone. In reinforced concrete, steel bars give strength when the concrete is stretched, and the concrete is hard and resists being squashed. Carbon fibre composites are strong and have a low density, so they are used in bicycle frames. Density is found from density = mass ÷ volume. When comparing materials quantitatively, use the same units for every value and compare them directly, for example density in g/cm3.