(a) Define density. What is the SI unit of density?(b) Define relative density. What s the SI unit of relative density?

(a). Density- The mass of a substance per unit volume is the density of the substance.

$Density=\frac{mass}{volume}$

The SI unit of density is $kg/m^3$.

(b). Relative density- The ratio of the density of a substance to the density of water is known as the relative density.

$Relative\ density=\frac{Density\ of\ substance}{density\ of\ water}$

Relative density is a ratio and it is unitless.