Heavy for Its Size: Density

Chemistry
Ages 11–14 · Grades 6–8
Hands-on experiment

What you’ll find out

Density is mass divided by volume, and it belongs to the material, not the object.
The volume of an irregular object can be measured by the water it displaces.
An object floats in a liquid when its overall density is less than the liquid’s.

No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.

Mass packed into a space

Hold a wooden cube in one hand and an iron cube of the same size in the other, and the iron is far heavier. Both take up the same space, but the iron packs much more mass into it. How much mass a material has in each unit of volume is called its density: mass divided by volume. A 1 centimetre cube of iron has a mass of about 7.9 grams, so iron’s density is 7.9 grams per cubic centimetre. The same cube of pine is about half a gram, and water is 1 gram. Density does not depend on how much you have: a nail and a heavy iron block are both 7.9. It belongs to the material, so it helps identify what something is made of.

Measuring an awkward shape

Finding the volume of a cube is easy: multiply length, width and height. But what is the volume of a stone or a key? Let water measure it. Pour water into a measuring cylinder and read the level, say 50 millilitres. Lower the stone in until it is completely underwater and read again: 58 millilitres. The stone has pushed aside 8 millilitres of water, so its volume is 8 cubic centimetres, because one millilitre is one cubic centimetre. If the stone’s mass is 20 grams, its density is 20 ÷ 8 = 2.5 grams per cubic centimetre, about right for many rocks. This works for any object that sinks and does not soak up water.

Liquids that stack

Density also decides what floats on what. Pour honey, then water, then vegetable oil gently into a tall glass and they settle into three layers: honey at the bottom (about 1.4 grams per cubic centimetre), water in the middle (1.0) and oil on top (about 0.9). Drop in small objects and each stops where it fits. A cork floats on the oil, and a grape sinks through the oil and water to rest on the honey. An object floats when its overall density is less than the liquid’s and sinks when it is greater. That explains why oil spills spread on the sea, why ice floats, and why a steel ship floats: its hull holds so much air that the ship as a whole is less dense than water.

Curriculum links: NGSS MS-PS1-2