Define Relative Density. What is its SI unit?

Hint: The ratio of a substance's density (mass per unit volume) to the density of a specified reference material is known as relative density, or specific gravity. For liquids, the reference is almost always water at its densest temperature (\[4{\text{ }}^\circ C{\text{ }}or{\text{ }}39.2{\text{ }}^\circ F\]) for gases, the reference is air at room temperature (\[20{\text{ }}^\circ C{\text{ }}or{\text{ }}68{\text{ }}^\circ F\]).

Complete answer:
If the relative density of a substance is less than one, it is less dense than the reference, if it is larger than one, it is denser. The densities are equivalent if the relative density is exactly one, that is, equal volumes of the two substances have the same mass. A substance having a relative density (or specific gravity) less than \[1\] will float in water if the reference material is water.

An ice cube, for example, with a relative density of \[0.91\] will float. A substance sinks if its relative density is larger than one.
\[\text{Relative density} = \dfrac{\text{Density of substance}}{\text{Density of water}}\]
Relative density is a ratio so it has no units.Because the density of substances varies with temperature and pressure, the temperatures and pressures at which the densities or masses were obtained must be specified. Because relative density normally relates to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products), fluctuations in density caused by pressure are usually ignored, at least where apparent relative density is being evaluated.

Note: Relative density can also be used to calculate a material's buoyancy in a fluid or gas, or to determine the density of an unknown substance based on the density of a known substance. Geologists and mineralogists frequently utilise relative density to determine the mineral composition of a rock or other sample. It is used by gemologists to aid in the identification of gemstones.