Density
The density of a substance is defined as its mass per unit volume.
Density (ρ) = mass (m)/ Volume (V)
Unit of density:
In the cgs system: gcm-3
In SI: kgm-3. (1gcm-3 = 1000 kgm-3)
Relative Density
The relative density (RD) of a substance is the ratio of the density of the substance to the density of water at 40C. From the definition, the relative density of a substance is given as
RD = Density of the substance / Density of water at 40C.
Thus, relative density is also defined as the ratio of the mass of the substance to the mass of an equal volume of water at 40C.