What is the mass per unit volume of a substance called?

Hint: Here we have basically been asked the ratio of mass per unit volume of a substance. So we will first see what does the quantity mass and volume means with a simple example for each quantity. Then we will define its ratio and see what it represents with an example.

Complete answer:
Mass is a fundamental property of all matter denoted by $m$. Mass is a measure of the amount of matter used up in making a substance. Which usually is measured in grams ($g$) or kilograms ($Kg$). Mass measures the quantity of matter in the substance regardless of both its location in the universe and the gravitational force applied to it. SI unit of mass is Kilogram ($Kg$). For example a person weighs $65\,Kg$ on earth that same person will weigh $10.725\,Kg$ on moon because there is no gravitational force acting on that person but mass of that person is same i.e. $10.725\,Kg$ whether it be on Earth or Moon.

Now moving on to the second parameter Volume. Volume can be defined as the space that a substance or shape occupies or contains. Volume is denoted by $V$. Volume is a three dimensional quantity enclosed by a closed surface or boundary. SI unit of volume is cubic metre $\left( {{m}^{3}} \right)$. For example square has no volume because it is a two dimensional shape which cannot hold any substance within it. Whereas, a cube has volume equal to $lengt{{h}^{3}}$ constituted by its three dimensions of equal length.

Now taking their ratio that is mass per unit volume $\left( \dfrac{mass}{Volume} \right)$ which means it should be a quantity which relates mass and volume of that object. The ratio of mass to the object's unit volume means how much mass can fit into unit volume which we know is the definition of density.

Hence, the mass per unit volume of a substance is known as density.


Note: Density is a unique physical property of an object. The more the density will be the more tightly matter will be packed together in the given substance. The SI unit of density is given as kilogram per cubic metre $\left( \dfrac{Kg}{{{m}^{3}}} \right)$.