Question

What is the SI unit of density?
$\text{A}\text{. kg }{{\text{m}}^{\text{-3}}}$
$\text{B}\text{. kg }{{\text{m}}^{\text{-2}}}$
$\text{C}\text{. k}{{\text{g}}^{3}}$
$\text{D}\text{. }{{\text{m}}^{3}}$

Answer 1

First, recall the definition of density and then use the mathematical expression for density to obtain its SI unit.
Mass of a substance divided by its volume is equal to the density of the substance. Density of a substance is very useful in determining purity of the substances.

Complete step-by-step answer:
Density of any substance is defined as mass of the substance per unit volume of it. In other words, we can say density is the specific mass of the substance. Usually $\rho$ is used to denote density.
Mathematically, It is mass divided by volume.
$\rho =\dfrac{m}{V}$
Where
$\rho=$density of the substance
$m=$mass of the substance
$V=$volume of the substance
SI unit of mass is kilogram ($kg$) and volume is cubic meter (${{m}^{3}}$). Therefore the SI unit of density is $kg\text{ }{{\text{m}}^{-3}}$.

So, the correct answer is “Option A”.

Additional Information:
Density of a substance varies with temperature and pressure.
With temperature, due to thermal expansion volume of a substance increases and as a result density of the substance decreases, for mass remains constant.
Usually density of a substance increases with increasing pressure because when pressure is applied on the surface of the substance its interatomic spacing decreases. But water shows exceptional behaviour. Water contracts on heating between ${{0}^{{}^\circ }}C$ and ${{4}^{{}^\circ }}C$. When water is cooled, its volume decreases continuously until its temperature reaches ${{4}^{{}^\circ }}C$ and after that its volume increases. As a result, its density increases until ${{4}^{{}^\circ }}C$ temperature on cooling. Hence we can conclude that water has maximum density at temperature ${{4}^{{}^\circ }}C$.
Pure water has a density of $1\text{ }gm-m{{L}^{-1}}$ at ${{4}^{{}^\circ }}C$ temperature.
Relative density is the ratio of density of any material to the density of some standard material. Usually water is used as the standard material.

Note: Student should not get confused between the terms relative density and density. Relative density is dimensionless and unitless quantity.
International system of units (abbreviated as SI) is an internationally accepted system of units at present. In SI, there are seven base units namely metre, kilogram, second, ampere kelvin, mole and candela. Other derived units can be written as a combination of any of these base units.