Different classes of material have different properties, and these can be compared using numbers. The values below are typical ones.
- Metals, such as steel: density about 7.8 g/cm3, melting point about 1500 °C, tensile strength about 500 MPa. They are strong, can be shaped and are good electrical conductors.
- Polymers, such as poly(ethene): density about 0.95 g/cm3, melting point about 120 °C, tensile strength about 30 MPa. They are light, flexible and electrical insulators.
- Glass: density about 2.5 g/cm3, softens at about 700 °C, tensile strength about 50 MPa. It is hard but brittle and is an electrical insulator.
- Clay ceramics, such as brick and porcelain: density about 2.4 g/cm3 and they stay solid above 1000 °C. They are hard but brittle and are electrical insulators.
- Composites, such as carbon fibre in a polymer: density about 1.6 g/cm3, tensile strength over 1000 MPa.
A composite is made of strong fibres held in a matrix. This gives it a very high strength for a low density, which is why it is used for aircraft parts and racing bikes.
Numbers let you make a quantitative comparison. Take three densities: steel about 7.8 g/cm3, glass about 2.5 g/cm3 and poly(ethene) about 0.95 g/cm3. Dividing one density by another tells you how many times denser one material is. Glass and clay ceramics are brittle, so they shatter when hit rather than bending.