The famous Mpemba effect – that in some situations, hot water can freeze faster than cold water – is a fascinating effect. What makes it even more fascinating is that no one seems to be able to explain it properly.
The graph below shows a trial that I performed, which shows some effect, i.e. the hotter water cools much faster than the cold water; however it appears that the cold water is still the first to reach 0°C. And thus lies much of the problem of describing the Mpemba effect – is freezing defined as reaching 0°C, forming a visible layer of ice on the surface, or becoming a solid block of ice? The experiments by Mpemba and Osborne used the time for freezing to start, but it may be easier to use a datalogger to monitor the temperature, rather than having to check on each sample every few minutes.
But my initial trial (below), which used the same water for each trial (I just allowed it to heat up again after it had been in the freezer) did not show the same rapid cooling of the higher temperature water. The coldest water (about 3°C) did take longer to reach 0°C than the 11°C water. Additionally, Mpemba found that water with an initial temperature of 25°C took the longest time to freeze out of any starting temperature, which could explain why it took so long in this trial. Hmm.
To monitor the temperature, I used an Arduino board, connected to a thermoresistor I salvaged from an old rice cooker. I calibrated the thermoresistor using water at different temperatures, and measuring the temperature as precisely as I could with a glass thermometer.
In the end, I ran out of time, as this experiment is so complex and there are so many factors to consider, but I would love to have another look at it one day.
If you’re interested (who wouldn’t be?), the Wikipedia page is a good place to start. Also, the article by Jeng (2005) contains a great commentary on possible explanations, and a review of different results.