See hot water rise

See hot water rise

This great experiment to show how hot water is lighter than cold water!

Ingredients and equipment that you will need

Hot and cold water
Four plastic cups. They need to be rigid, squashy cups won’t work for this and you need to be able to see through them.
Food colouring
Small piece of thickish plastic e.g. from a strong plastic bag.
A large tray

Method and further information

Do this on the tray because it’s messy, and in the kitchen or the bathroom or even better outside!

Fill two cups with hot water (bath temperature, no hotter) and two others with cold water. Colour them different colours so you know which are hot and which are cold (one can be just water if you’ve only got one food colouring)

Put a plastic bag over one of the cold ones and quickly turn it upside down on top of a hot cup. Carefully pull the plastic out. Do the same with a hot cup on top of the cold one.

When you heat something up you give it energy, so it expands and gets lighter. Amazingly hot water is lighter than cold water.

When the hot water is at the bottom they mix because the hot water is lighter, so it rises up. When the cold water is at the bottom they don’t mix because the hot water, being lighter, stays at the top.

Be careful – you should ask an adult to help you with these experiments. Do your experiments in old yoghurt pots, jam jars or bottle tops. Don’t use cups or glasses in case someone thinks it’s a drink.