1Mass and volume
Blocks of the same size can have very different masses. Density tells you how much mass is packed into each cubic centimetre of a material.
density
Lesson 29.1 notes
Density
Mass divided by volume, how to measure it, and why things float or sink.
2Working out density
Density = mass ÷ volume. This aluminium block has a mass of 54 g and a volume of 20 cm³, so its density is 54 ÷ 20 = 2.7 g/cm³.
3Cut it in half
Cut a block in half and its mass and volume both halve, so its density stays the same. Density belongs to the material, not to one object.
4Measuring a box shape
For a box shape, measure the length, width and height with a ruler and multiply them to get the volume. Then find the mass on a balance.
volumemass
5Odd shapes
Lower a stone into a measuring cylinder: the rise in the water level is its volume. Or use a displacement can and measure the water that overflows.
displacement can
6Float or sink
Water's density is 1 g/cm³. Anything less dense than water, like wood or oil, floats on it. Anything denser, like stone or iron, sinks.
7Liquids in layers
Liquids that don't mix easily settle in layers, densest at the bottom. A solid sinks until it reaches a liquid denser than itself, then floats there.
Explore: Density lab
Pick a material and change the size of its block: the mass and volume change, but the density doesn't. Then drop it in water to see if it floats.
Common mistake: Dense doesn't mean heavy
A big log is heavy, but wood isn't dense; a gold ring is light, but gold is very dense. Density is the mass in each cm³ of a material, whatever its size.
Remember
- density = mass ÷ volume, measured in g/cm³ or kg/m³ (1 g/cm³ = 1000 kg/m³).
- Find a box shape's volume from length × width × height, and an odd shape's from the water it pushes aside.
- Water's density is 1 g/cm³: anything less dense floats on it, and anything denser sinks.
Going further at GCSE