The Colour Exchange

Try this awesome experiment at home! First look at the video below and see if you can predict what science is going on!

HERES A CLUE: you’ll need to think about the words density and convection.


2 identical drinking glasses, 2 different colours of food colouring (ideally ones that are not similar), some card (cereal box thickness), scissors, a bowl or a nearby sink


  1. Fill one of your glasses with hot water from the tap and colour it red.
  2. Fill your other glass with cold water and colour this blue.
  3. Place your piece of cardboard over the top of your cold water.
  4. Watch out…the next bit needs to be quick!
  5. Flip the cold water glass upside down keeping the cold water inside the glass.  Gently place this glass on top of the glass of hot water. (Give this stage a try over the sink first to check you can manage it!)
  6. Carefully slide the cardboard out from between the two glasses and don’t worry if some water drips out.
  7. Watch what happens to the hot water!
  8. Now try this the other way round

The science:

When we heat up water we give it energy. This means that the water molecules have more energy to move around faster. As this happens the water takes up more volume or space which gives it a lower density. The hot water has a lower density than the cold water, so rises rapidly to the top where it mixes with the cold water. When we do it the other way round the hot water doesn’t mix, it stays floating on top.

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