Density is the mass--that is, the amount of matter--in a unit volume of any substance. The density of a substance is found by dividing its mass by its volume. The density of a liquid or solid is measured in grams per millilitre. The density of a gas is measured in grams per litre.

The concentration of a substance in a solution can be determined by measuring the density of the solution. Density measurements are useful in identifying minerals and other solids. In addition, the molecular weight of a gas can be calculated from its density .

The density of a liquid can be determined by measuring the mass needed to fill a container of a known volume. In most cases, a device called a pycnometer, which has a precisely known volume, is used for this purpose. An instrument called a hydrometer is also used to determine the density of a liquid.

The density of a regularly shaped solid is determined by simply measuring the object's mass, calculating its volume, and dividing the mass by the volume. The density of an irregularly shaped solid is determined by submerging it in a known quantity of liquid and measuring the volume of the liquid displaced. The volume of the displaced liquid equals the volume of the solid. The mass of the object is then determined and is divided by the volume.

The density of a gas is difficult to measure because it is extremely low and changes greatly with variations in temperature and pressure. The mass of a gas can be determined by subtracting the mass of an empty container from the mass of the same container when filled with the gas. The volume of the container can simply be found by measuring the amount of water (using a graduated flask) that the container holds.

The specific gravity of a substance is related to its density. Specific gravity is the ratio of the mass of a given volume of the substance to the mass of an equal volume of water. It is found by dividing the density of the substance by the density of water at either 4 °C or 20 °C.

A quick look at Density

It is the simple ratio between mass and volume of a substance.
CGS unit of density - gm/cm3
MKS unit of density - kg/m3
1gm/cm3 = 1000 kg/m3
Finding density of a regular solid
Mass = 50kg , volume = length * breadth* height = 20m
Density = mass/volume
= 50/20
=2.5 kg / m3

Finding density of an irregular solid
Calculate their masses
Volume of the body = (y-x)cm
Density = mass/volume = m/y-x

X cm3 y cm3

Determination Of Density Of A Liquid
Mass of an empty bottle - m1 gm
Mass of bottle with water - m2 gm
Take out the water dry the water and fill the bottle with the liquid whose density is to be taken.
Mass of bottle filled with liquid - m3 gm
Mass of water - (m2 - m1) gm
Mass of liquid - (m3 - m1) gm
Density of water = 1gm/cm3
Density = mass / volume v = m/d
Volume of water = m2 - m1 / 1gm/cm3
Volume of liquid = volume of liquid (as they were contained in the same vessel)
Volume of liquid = m2-m1 cm3
Density of liquid = m3-m1 / m2-m1

Finding density of air
Take an empty (vacuum) vessel flask and note its mass = m1 gm
Mass of flask filled with air = m2 gm
Volume of air filled in flask = V cm3
Mass of dry air = (m2 - m1) gm
Density of air = m2-m1/V