Melting ice cubes

This was a very simple activity, but it was apparently entertaining enough to be asked for again several times...

We froze some water to make big ice cubes. They weren't cube-shaped as we've got a silicone container meant for portions of baby food which takes about 50ml in each little round-bottomed pot.  However, you could do this with a normal ice cube tray for a quicker activity or use any freezer-safe container to get bigger bits of ice that last longer. We added blue food colouring to the water first to make things easier to see and left to freeze overnight.

Freezing the water was clearly memorable as immediately after breakfast the next morning, the small boy asked to play with his ice cubes. I got out two big plastic boxes, and filled them (not entirely full as toddlers are clumsy...) with water. One had tap cold water and the other hot water from the tap.  The small boy enjoyed sticking his hand into each one several times to feel the different temperatures,  Then, into each box at roughly the same time, we put a big ice cube.

Checking the temperature of the water to remind himself which one was warmer!

I'd asked the small boy to tell me what he thought would happen. He said it would sink, and it would melt (not sure where he's learnt this, possibly from ice cream which he discovered this Summer...?). When I asked about whether it would melt faster or more slowly in the hot water, he told me (correctly) it would be faster. I like asking him to make a prediction when we do new things as it makes him engage a bit more with the activity and it also gives me a better understanding of what he knows (or misunderstands) and therefore what to try and explain to him whilst we do things.

When the ice cubes hit the water, there was a loud cracking noise and big cracks appeared on the surface as the ice quickly changed temperature on the outside of the ice cube.  The small boy thought this was really exciting! Then the ice cubes bobbed back up to the surface and he exclaimed in surprise 'they float'!  I didn't try to explain why as I couldn't think of a simple explanation.

The ice melted faster in the warm water

He watched transfixed as the ice in the hot water melted over about 5 minutes, poking the ice cube and testing the water temperature every minute or so. The blue food colouring (which interestingly freezes in the centre of the ice cube, presumably because it remains mobile in the liquid water as the ice starts to form on the outside of the ice cube, and then gets trapped in the structure of the ice as the whole thing freezes) means that as the ice melts, the blue colour sinks with the cold water melting from the ice to the bottom of the container before it mixes with the rest of the water.  We watched the stream of blue towards the bottom of the boxes, and I explained that cold water sinks to the bottom.  He didn't point out that the cold water was warmer than the ice cube and ask why it floated, but I'd have had to try and explain why ice is less dense than warm water, and cold water is more dense than warm water if he had!

Blue food colouring going down with the cold water from the melting ice

We then had a short conversation about temperature.  He's clearly got the idea that some things are colder or warmer than others, and I tried to explain how we use numbers to describe how cold or warm things are i.e. their temperature.  I'm not sure he really understood, but he has since been able to tell me that ice melts at zero degrees (Celsius) although I suspect this is because he's fascinated by the episode of Numberblocks which features zero.  I'll have to try something else where we can talk a bit more about temperature soon.

We have since done this activity another couple of times at his request.  I didn't think it'd have the potential to be worthy of a repeat, but I have been proved wrong.  On one of the repeats after the ice had melted and the boy had lost interest, I had a go at trying to put red food colouring in near boiling water at the bottom of the tank to see if it would rise to the surface, but I couldn't find a way to introduce the hot water without creating a lot of turbulence in the tank, so it didn't work nicely (tried a children's medicine syringe and opening a lid on a small pot).  Any ideas on how to do this more successfully are very welcome - please leave a comment below!