Specific gravity, also called relative density is the ratio of the density of a substance to that of a standard substance. In this case, the standard substance is pure water.
Simply, specific gravity defines whether an object will sink or float in water.
Specific gravity is used to define the density of a liquid as compared to the density of an equal volume of water at a specified temperature.
The specific gravity has no unit because the units of the numerator and the denominator are the same.
Formula to calculate specific gravity.
The density of liquid X is 21g/ml while the density of pure water at 4°C is 1g/ml. Determine the specific gravity of liquid X.
Therefore, the specific gravity of liquid X is 21.