Density

Density

The density of a substance tells how much matter is contained in a certain volume of it. Usually the standard volume for comparing is 1 cm³ or 1 m³ .

It is measured in units of kgm^-3 (or you may also see gcm^-3).

A substance that has a density of 11 gcm^-3 means that 1 cubic centimeter (i.e. a volume = 1 cm³ ) of the substance has a total mass of 11 g packed into it.

The table above shows similar information for other objects and we can say that 1 cm³ of object D has a mass of 0.95 g.

NOTE: Comparing the masses of equal volumes of substances is equivalent to comparing their densities.

The density of a substance is defined as the mass per unit volume of the substance. It is given by the equation

density = mass / volume

In symbol form

ρ = m/V

Relative Density

This is the number of times a substance is more dense (heavier) than water. The density of water is known to be 1.0 x 10³ kg/m³. The density of lead is 11 x 10³ kg/m³. Therefore lead is 11 times more dense than water and the relative density of lead is 11. Relative density has no units.

relative density = density of substance / density of water

If we compare the masses of equal volumes of a substance and water, we are essentially finding the relative densities of the substance. Here is an example:

The table shows that 5cm³ lead is 14 times the mass of 5 cm³ of water; (since 70 / 5 = 14)

Therefore the relative density of lead is 14.

We can now write the following equation.

relative density = mass of certain volume of substance / mass of equal volume of water

Density Towers

Liquids can float above or sink below water based on their relative densities. The video below illustrates this.