(a) What is the difference between the density and relative density of a substance ?
(b) If the relative density of a substance is 7.1, what will be its density in SI units ?

(a). Difference between the density and relative density:

Density	Relative density
Density is the mass of a substance per unit volume.	Relative density is the ratio of the density of the substance to the density of the water.
$\boxed{density=\frac{mass}{volume}}$	$\boxed{Relative\ density=\frac{density\  of\ the\ substance}{density\ of\ the\ water}}$

(b). The relative density of the substance $=7.1$

We know, $Relative\ density=\frac{density\  of\ the\ substance}{density\ of\ the\ water}$

Therefore, $7.1=\frac{density\ of\ the\ substance}{1000\ kg/m^3}$    [density of the water is $1000\ kg/m^3$]

Or $density=7.1\times1000\ kg/m^3$

Or $density=7.1\times10^3\ kg/m^3$

Therefore, the density of the water is $7.1\times10^3\ kg/m^3$.