I first saw the demonstration in the video below done by Clare Thomson at the “Best of PhysEd” lecture at the ASE Conference in 2010. Ever since I tried to make various versions of it, from using two very tall gas columns, filming it with high frame rate cameras, etc. But today I think I have made a really interesting variation of this really nice demo. The video below was made this morning in my kitchen.
Recreating this demo is very simple and I strongly recommend you do it with your classes, because the colours in the video don’t really reflect what you can see with your naked eye. I used water beads that I previously immersed in water containing blue food colouring for the cold water beads and red food colouring for the hot water beads. You will need to leave them in dyed water for about 8 hours. Then, I put cold water in the glass with blue beads and boiling water in the glass with red beads. When you mix cold and hot water with the cold water at the top, the red bead (much hotter) will rush upwards and the blue beads (much colder) will fall downwards. As the two types of beads swap places you have a nice simulation of what happens to the particles from hot and cold water, i.e. with more or less kinetic energy, when they mix. You have a very visual representation of a convection current forming in the two glasses. There is a limitation though, in fact, you can see that after a while the red beads begin to fall and collect at the bottom on top of the blue beads, but this is still quite effective at making the point that they have swapped places.