Density of a substance is defined at its mass per unit volume. It simply tells us how much mass is packed in a unit volume of a substance.
Density = mass/volume
Relative density of a substance is defined as the ratio of the density of the substance to the density of water at 4 degrees Celsius. Another term used for relative density is specific gravity.
Relative density of a substance = Density of substance/Density of water at 4 degrees Celsius...