Explain the specific density or relative density of a substance.

The relative density of a substance is defined as the ratio of its density to the density of water at $4^{\circ} C$.
Relative density $= \frac{\text{Density of substance}}{\text{Density of water at } 4^{\circ} C \text{ temperature}}$
For example: The density of aluminium is $2.7 \times 10^{3} \ kg \ m^{-3}$. The density of water at $4^{\circ} C$ is $10^{3} \ kg \ m^{-3}$. Hence,the relative density of aluminium is $2.7$.
Relative density is a positive,scalar,and dimensionless physical quantity.
(Density of substance = Relative density $\times$ density of water at $4^{\circ} C$ temperature)
The densities of some common fluids are displayed in the table below:

Fluid	$\rho \ (kg \ m^{-3})$
Water	$1.00 \times 10^{3}$
Sea water	$1.03 \times 10^{3}$
Mercury	$13.6 \times 10^{3}$
Ethyl alcohol	$0.806 \times 10^{3}$
Whole blood	$1.06 \times 10^{3}$
Air	$1.29$
Oxygen	$1.43$
Hydrogen	$9.0 \times 10^{-2}$
Interstellar space	$\approx 10^{-20}$