Here is a lovely classroom demonstration that I saw at the ASE Conference 2010 in Nottingham. The demo was part of the Physics Education Lecture, which displayed the best of the PhysEd magazine. I really learned a lot and was well impressed by the quality and creativity of the demonstrations, activities and workshops proposed by the Institute of Physics. As one of the IoP Network Coordinators I was very proud to be part of the Institute and see how many outstanding workshops and lectures they put together for the event. Apparently, the IoP did the majority of workshops and they were all free of charge, although the conference was organised by the ASE.

Anyway, coming back to our demonstration. At the lecture it was shown using two small glasses, so when I went back to my lab I thought; “What would happen, if I use two very tall columns of water? And this was the result!

So, why won’t the two liquids mix?

I put cold water in the bottom column with some blue food colouring and boiling hot water in the top column with some red food colouring. The tricky bit is how to turn the top column upside down, as it is really hot and heavy, but it was well worth it! So, I put a sheet of paper on the top and then carefully turned it upside down (you might need a helper to do this). Then, I placed the top column on bottom one and as you can see, and unlike what the kids would expect, the red and blue water don’t mix. They actually stay unmixed for a very long time (over an hour at least).

But how do we explain such an effective phenomenon? Well, the hot water is less dense than the cold water at the bottom, as its particles have more kinetic energy, hence moving further apart from each other. The result is that we have two liquids of different density, with the less dense one at the top, which therefore will float on top of the denser cold water. It is a bit like having oil and water, you can tell your students!

This is a really nice demonstration that will really help your pupils to understand that hot liquid rises and cold liquid falls. It’s not only very memorable, but it also shows quite clearly that in heat convection currents it’s not the “heat” that rises, but the hot liquid, or gas.