Density of hot and cold water

After my son enjoyed an activity about temperature, I thought he might be entertained by a colourful demonstration of hot water being less dense than cold water.  It's potentially messy, so we did it outside, although our timing wasn't great and it started to rain as we were finishing.  I made it a bit more complicated than is actually necessary as I wanted some opportunities for my son to get involved with measuring temperature and pouring, rather than just watching.

Ready to go

The first step is to take four identical glasses, and fill two of them with cold water and two with hot water.  We used some blue colouring in our cold water, and as we had some room-temperature blue coloured water already made up for other things I gave him a little of this in each of two glasses and a jug of cold water to pour into them. He measured the temperature of the water in the jug (5 degrees Celsius) and then poured the water across, taking care not to spill it.


The process for the hot water was the same, with him adding hot water (well, warm, it was 50 degrees Celsius to make it safe for my son to pour, but slightly hotter water would probably work a bit better) to a little room temperature red water.  You could simplify this and just add warm water to red food colouring and cold to blue (or indeed any other colours you've got to hand, we just used these as they tend to be associated with hot and cold).  He didn't want to measure the temperature of the water once it was coloured, so we moved on, but I would guess it was about 10 degrees Celsius for the cold and about 40 degrees for the warm.

Hot and cold water, with the covers in between

The next step is tricky and not one for a 3 year old to do without a lot of help! You need something to cover the top of a water-filled glass which you can hold on tightly so you can turn it upside down and place it on top of another glass without spilling the liquid. We had a couple of lids off stacking crisp (Pringles) tubs which I cut with scissors to remove most of the rim, leaving a little in place so it is easier to grip later on.  For one pair of hot/cold water glasses, we put the hot one on top and for the other, we put the hot one on the bottom.  As you can see from the pictures, a bit of spillage happened despite my best efforts.

Once you've got the glasses stacked with the cover still in between them, you need to pull it out carefully so that no liquid spills.  The Pringle lid was ideal for this as it was a little larger than our glasses, and the little 'handle' I left on the edge meant 3 year old fingers were able to help.  I held the glasses in place for the first pair (hot on the bottom, cold on the top) and he pulled the cover out.  We saw all the liquid quickly turn purple!  The hot and cold water mixed because the most dense was on top and the least dense underneath.  My son wasn't so keen on pulling the cover out for the other pair, so I did it, and there was a little mixing in the middle, but the red layer stayed at the top and the blue layer at the bottom, because the most dense liquid was already at the bottom.

The end result

It's fair to say that my son could have been more impressed. We didn't talk very much about density because he lost interest and it was starting to rain.  He was keener when we poured them away and I tipped the pair of glasses with the separated layers sideways and the blue moved to the bottom again and the red to the top.  I think this is one to come back to again when he's a bit older, and if he's interested when we do it again it might be fun to observe what happens when the cold water warms and the hot water cools.