This hot weather has got me all of a fluster, and something has really been bothering me. Hopefully this little experiment will cool me down…so why on earth does hot water sometimes freeze faster than cold water?
I’m glad it’s not just me who’s got rather confused with this occurrence, because it’s been baffling brilliant minds for centuries, and not even Aristotle could suss this one out. It’s now become known as the Mpemba effect, after a student in a Tanzanian high school, Erasto Mpemba, quizzed his teacher about the phenomenon in the 60s. His teacher of course did not have the answer to this conundrum. Years later, the curious Mpemba continued experimenting, and teamed up with a university professor, Dr Osborne, to try and find out the cause. They published all of their experiments in 1969, but alas, still had no answer.
Well, now the Royal Society of Chemistry wants to find out the cause once and for all, and they’re offering a £1,000 prize to anyone who comes up with a solution. So all you scientists, and budding scientists out there, let’s get to work!
To carry out your own simple experiment with your children you’ll need:
- A measuring jug
- Hot water
- Cold water
- Two paper or polystyrene cups of the same size.
First things first, label one cup ‘hot water’ and the other ‘cold water’. Measure out equal amounts of both hot and cold water, and pour into the respective cups. You’ll then need to put each cup in the freezer at the same time.
Keep checking back every 10 to 15 minutes, and record what you see. Which one freezes faster? Ask your children to analyse the ice cubes; are they the same in size and appearance?
The Mpemba effect only occurs under certain circumstances, so whether or not you encounter it, you’ll be able to explain to your children what factors affect water freezing:
Water temperature. Whatever temperature the water was when you put it in the cup will affect how it freezes; how hot was your hot water, and how cold was your cold water?
Conduction is the way in which heat transfers from one object to another. So, depending on how easily heat transfers through the material the water is stored in (in this case a paper/polystyrene cup), will affect how quickly the water freezes.
Evaporation. The more water that evaporates from the cup, the quicker it’ll cool down, as when the molecules evaporate they take the energy away from the liquid, which was keeping it warm.
You can also ask children to consider what other factors might affect the experiment, like the temperature of the freezer, the amount of water in the cup, and the size, shape and material of cup you use.
You’ve got until 30th July to submit your entries, so get experimenting!
Awesome sources: http://www.guardian.co.uk/science/blog/2012/jun/26/hot-water-freeze-faster-cold-water
Professor Smarty Pots 
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