The density of a substance is defined as mass of a unit volume.
The unit of density is kilogram per metre cube (kg m–3).
The density of a given substance, under specified conditions, remains the same.
Therefore the density of a substance is one of its characteristic properties. It is different for different substances. For example, the density of gold is 19300 kg m-3 while that of water is 1000 kg m-3. The density of a given sample of a substance can help us to determine its purity.
It is often convenient to express density of a substance in comparison with that of water.
The relative density of a substance is the ratio of its density to that of water:
Relative density = Density of a substance / Density of water
Since the relative density is a ratio of similar quantities, it has no unit.
Relative density is a scalar quantity