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This lesson looks at density.
This page includes:
two videos explaining what density is and how we can work it out.
one activity to try at home.
Density describes how closely packed the particles are in a solid, liquid or gas.
The density of an object or substance is its mass divided by its volume: 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 densities of some everyday substances are:
Steel has a density of 7.82 g/cm³
Water has a density of 1.00 g/cm³
Air has a density of 0.0013 g/cm³
These values show that steel (solid) is the most dense while air (gas) is the least dense.
The particles in solids are very close together. They are tightly packed, giving solids high densities.
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.
The particles in gases are very far apart, so gases have a very low density.
Measuring the 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.
Working out the density
Watch this short clip which explains how we can work out the density
Let's work through an example together
There are lots of ways to try out your scientific knowledge.
Try this activity to get you started.
Have a go working out the density with this activity:
There's more to learn
Have a look at these other resources around the BBC and the web.