Density

Density is the mass per unit volume. This means that the density of any solid, liquid or gas can be found by dividing its mass in kilograms by its volume in cubic metres.

Density can be found using the equation:

Density = mass / volume

• 
• The unit for density is kg m^-3
• The density of water is approximately 1000 kg m^-3 and air is approximately 1.3 kg m^-3

Densities of solids, liquids and gases

When most substances change from a solid state to a liquid state their volume does not change much. This is because the particles stay approximately the same distance apart. This means that the density of a substance, say iron, does not change by much when it melts.

When a liquid changes into a gas, the spacing between the particles increases by a factor of about ten. As such, because the volume of a substance is length × breadth × height, the volume will increase by a factor of 1000.

As a result, the density of a substance decreases by a factor of approximately 1000 when it changes from a liquid to a gas.