What is density?
Density is the amount of mass in a certain volume of a substance. It tells us how tightly the matter is packed together. A material with high density has a lot of mass squeezed into a small space, like iron. A material with low density has little mass for its size, like a sponge.
Key idea
density = mass ÷ volume
The common unit is g cm⁻³ (grams per cubic centimetre). The SI unit is kg m⁻³.
Calculating density
To find density, divide the mass of an object by its volume. For example, if a block has a mass of 20 g and a volume of 4 cm³, then:
density = 20 g ÷ 4 cm³ = 5 g cm⁻³
Example
A stone has a mass of 60 g and a volume of 20 cm³. Its density = 60 ÷ 20 = 3 g cm⁻³.
Floating and sinking
Density explains why objects float or sink in water. The density of water is about 1 g cm⁻³.
- An object less dense than water (below 1 g cm⁻³) will float, like wood.
- An object more dense than water (above 1 g cm⁻³) will sink, like a stone.
Remember
- density = mass ÷ volume, measured in g cm⁻³.
- Higher density means more mass in the same space.
- Less dense than water floats; more dense than water sinks.