Density is used to make a fair comparison between materials.

Key point

Density is the mass per unit volume of a material.

i.e. density is the mass of 1 cm3 or 1 m3 of a material.

The density of aluminium is 2.7 g/cm3 – this means that 1 cm3 of aluminium has a mass of 2.7 g.

All matter contains particles.

The difference between the different states of matter is how these are arranged:

  • in a solid - particles are packed close together in a regular structure. They vibrate about fixed positions;
  • in a liquid - particles are packed close together but are not fixed in position – they are free to move past each other;
  • in a gas - particles are very far apart and move randomly in all directions.
Solid cube: side length 1 unit, 64 particles in tightly packed lattice. Liquid cube: 1 unit, contains approx 30 particles. Gas cube: side length 10 units contains 15 scattered particles.Relative particle density for a solid, liquid and gas

Changing the temperature or the state of a material will change its density

Normally, when solids are heated their density decreases.

The spacing between the particles increases, but there is no change of mass.

So, a bigger volume has the same mass which means the density is smaller.

Similarly, when liquids evaporate the density decreases as the spacing between particles increases greatly.

The densities of some common substances

MaterialDensity in grams per cubic centimetre (g/cm³)
Aluminium2.7
Iron7.9
Ice0.9
Water1.0
Air0.0013