The Mysterious Mpemba Effect

The Mpemba effect is quite well known though maybe not by name outside of physics. It is the idea that hot water will freeze faster than cold water. I can remember a competition held by the Royal Society of Chemistry four years ago that had the goal of finding an explanation for the Mpemba effect. But slowing down might be required to explain where this almost paradoxical claim came from. The story I heard and I believe to be in the most part true is that a boy called Erasto Mpemba back in the sixties was creating ice cream as part of a class food project. He was running behind schedule and so instead of letting his mix cool before putting in the freezer he put it in still hot. He was surprised to find his ice cream was ready before his classmates who had put their cold mixtures in before him and at the same time as him. Apart from the eponymous example there are many recorded examples of this happening throughout history even as far back as Aristotle. The explanation I believe to be most valid is that the hot water produces convention currents that can still be maintained even when the water cools. This means the rate of cooling is always faster in the hotter container. Other theories talk about how cold water freezing causes a layer of ice over the surface which can insulate the water below slowing the cooling rate, but in hot water ice forms around the base and sides first so this insulation doesn’t get a chance to be made. A paper has just been released that seems to come to most simple of all however: the Mpemba effect doesn’t actually exist. Generally the Mpemba effect is seen more in home based freezing methods rather than in laboratories. The empirical studies that do show the Mpemba effect only ever find the difference in freezing time to be very small and many can’t find any evidence for it at all. It is possible that under the strict controlled conditions the effect is reduced to nothing in which case the source must be some kind of rudimentary action somewhere. This result does raise an interesting question; if an effect is commonly observed in nature but can’t recreated or observed in laboratory conditions what conclusion is science supposed to pull?


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