# Density

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## Density

We know that some materials are light while some are very heavy. For instance, a cube of polythene will weigh a lot less than the same size cube of steel.

We can systematically compare amount of matter in different materials by defining a property called **density (r).**

**Density = mass per unit volume**

**Where,**

M is the mass of an object, and V is its volume.

**Units of ρ: kgm ^{-3}.**

* Tip:* From the knowledge of density and molar mass of a solid or liquid, it is possible to estimate the average volume occupied by each molecule in a substance (V

_{molecule}) by using the following formula.

**Where,****M _{mole}** its molar mass, and

A = Avagadro's number = 6.023 x 10^{23}.

Approximate molecular spacing (i.e. distance D between adjacent molecules) in the material is given by the cube root of the volume of the molecule.

D = (V_{molecule})^{1/3}.

* Additional Definitions:* Molar mass of a material = mass of one mole of the material.

Mole of a material = amount of material containing 6.023 x 10^{23} molecules.