Density

The density of an object or substance is its mass divided by its volume:

curriculum-key-fact
density = mass ÷ volume

The units of density depend on the units used for mass and volume, but are usually:

  • g/cm3 (if mass is measured in g and volume in cm3)

The more dense a substance is, the heavier it feels for its size. The table shows three examples:

SubstanceDensity in g/cm³
Steel7.82
Water1.00
Air0.0013

Notice that the solid (steel) is the most dense, the gas (air) is the least dense, and the density of the liquid (water) is in between.

Solids

The particles in solids are very close together. They are tightly packed, giving solids high densities.

Liquids

The particles in liquids are close together. Although they are randomly arranged, they are still tightly packed, giving liquids high densities. The density of a substance as a liquid is usually only slightly less than its density as a solid. For example, the density of solid aluminium is 2.72 g/cm3 and the density of liquid aluminium is 2.38 g/cm3. This means that liquid aluminium floats on top of solid aluminium.

Water is different from most substances: it is less dense as a solid than as a liquid, because its particles move apart slightly on freezing. This is why ice cubes and icebergs float on liquid water.

Gases

The particles in gases are very far apart, so gases have a very low density.

Measuring density

You need to know two things to measure the density of a substance:

  • the mass of a sample of some of it
  • the volume of that sample

The mass is measured using a balance. The volume of a liquid is easily measured using a measuring cylinder. The volume of a solid can be measured by:

  • measuring the side of a cube or block of the substance, then using mathematics to calculate its volume, or
  • using a displacement can (also called a eureka can) – the sample is lowered into a container of water and the volume of water it displaces or pushes out of the way is the same as the volume of the object