Question

Why is density a derived unit?

Answer 1

A derived unit or a derived quantity is some physical attribute that is obtained by the mathematical relationship of two or more pre-existing fundamental physical quantities. Thus, such quantities depend on the changes that take place in the physical quantities from which it is derived.

Complete answer:
Density is a physical quantity that is measured as a ratio of mass and the volume occupied by any form of matter. The formula of density can be written as follows:
$density = \dfrac{{mass}}{{volume}}$
Density gives us the idea of how compactly a certain substance is dispersed into a fixed volume. Higher densities come from higher amounts of substances being held in smaller volumes. Densities are temperature dependent.
Any physical quantity that has units defined on the basis of a naturally occurring substance is called a fundamental physical quantity. Examples of fundamental physical quantities include mass, volume, length and temperature.
Density is a mathematical combination of two fundamental physical quantities and therefore its units are $kg{m^{ - 3}}$ which include the ratio of standard units of mass and volume.
$\Rightarrow$ Thus, Density is a derived unit as it cannot be measured in the form of a pure unit with complete independence and is actually expressed as a ratio of units of two fundamental quantities.

Note:
Even though fundamental quantities like temperature do not find their place in the formula of derived quantities like density, yet density values of various substances change with the change in temperature that takes place.