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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.

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