Question

Why reltive density is called specific density

Answer 1

First question arises is what is density , Density is the mass of 1 ml of any substance (ignore SI unit).

Now in fluid mechanics, specific gravity is defined as the density of any substance relative to water which is kept at 4°C, if we take gases the temperature is taken 25°C and (pressure is taken 1 atm).

The reason for fixing the temperature is that water achieves maximum density at that particular temperature.

Relative Density is the density of the substance taken relative to the standard fluid (in most liquids it is taken as water) and for gases it is air.

Answer 2

Your answer ...

Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the densityof a given reference material. Specific gravity usually means relative densitywith respect to water. ... A substance with a relative density greater than 1 will sink.
Therefore it is called specific gravity ...