What is Density?

Density can be a tough concept to grasp. Density refers to how much mass there is in a particular space. Imagine a drawer full of socks. It has a certain density. If you take a sock out, the density of the drawer changes. This is because the mass of the socks has changed, but the volume of the drawer has stayed the same.

Generally the more dense an object is, the heavier it feels.

Density = Mass divided by volume

The units used vary depending on the units of the mass and volume used for the calculation.

If the mass was measured in kg and volume in cm³ the units for density would be in kg/cm³

Something else that can be confusing is mass. People often talk about weight when they mean mass and vice versa.

Mass is a measure of how much matter an object is made up of. The mass of an object remains constant unless you add to or remove part of it.

The weight of an object depends on gravity. Your weight would be less on the moon as there is less gravity, but your mass would stay the same.

There are lots of very cool demonstrations you can do to demonstrate density in a fun, hands on way.  

Density Demonstrations

How to make a Density Jar

Image taken from This IS Rocket Science

What you need to make a density column

Vegetable or sunflower oil

Water

Food Colouring

Washing up liquid

Honey

Golden Syrup

LEGO or other small objects.

Instructions

Pour some water carefully into a glass or jar.

Very carefully add about the same volume of cooking oil on top.

Carefully drop couple of small objects into the mixture and observe what happens. Can you find an object to float on each layer?

What happens if you shake it the jar? You should find that the oil and water mix up and then separate again.

To make a a density column with more layers like the one above, you need lots of different liquids of different densities.

We used: honey, golden syrup ( or corn syrup ), washing up liquid ( dish soap ), water and food colouring, vegetable oil in that order.

Density of Liquids

Each of the liquids have a different mass of molecules or different numbers of parts squashed into the same volume of liquid, this makes them have different densities and therefore one can sit on top of the other – the more dense a liquid is the heavier it is.

Objects and liquids float on liquids of a higher density and sink through liquids of lower density. The LEGO brick  sinks through the oil but floats on the water, while the coin sinks through both. The coin is therefore more dense than both the oil and the water. The plastic bug floating on the oil is less dense than both.

More Experiments

Lava lamps are a fun experiment to demonstrate the fact that oil and water don’t mix.

Show your friends a science magic trick and fantastic density demonstration. This uses salt to change the density of one colour of water.

We also have a pirate themed density bottle. Pirate treasure is perfect for hiding in the layers.

Did you know that if you reduce the density of an object that sinks in water it will float? We did this with a heavy ball and used bubble wrap to make it less dense.

You can also use the same logic to make a lemon sink….