What is meant by relative density of a substance ? The relative density of mercury is 13.6. What is its density in SI units ? Given that density of water is 1000 kg m-3

It is defined as the ratio of the density of the
substance to the density of water at 4°C.
The density of mercury can be obtained from the expression
RD = density of mercury/density of water , hence
Density of mercury = RD x density of water = 13.6 x 1000 = 13.6 x 103 kg m-3