Density of Materials — AQA GCSE Physics
Density compares the mass of a material to its volume. It explains why some objects float and others sink.
What is density?
Density is the mass per unit volume of a material:
$$\rho = \frac{m}{V}$$
- ρ = density (kg/m³ or g/cm³)
- m = mass (kg or g)
- V = volume (m³ or cm³)
A dense material has a lot of mass packed into a small volume.
Density and the particle model
- Solids are usually the most dense — particles are closely packed.
- Liquids are slightly less dense.
- Gases are the least dense — particles are far apart.
Required practical
To find the density of a material:
- Measure the mass with a balance.
- Find the volume:
- regular solid → measure dimensions and calculate,
- irregular solid → water displacement (eureka can or measuring cylinder),
- liquid → measure a known volume in a measuring cylinder.
- Calculate density = mass ÷ volume.
Worked example
A block of mass 240 g and volume 30 cm³: density = 240 ÷ 30 = 8 g/cm³.
Exam tips
- Learn ρ = m/V and rearrange it.
- Find the volume of an irregular solid by water displacement.
- Convert units carefully (1 g/cm³ = 1000 kg/m³).
- Link density to particle spacing (solids densest, gases least).