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HOT water sometimes freezes faster than cold water - but why? This peculiar phenomenon has baffled scientists for generations, but now there is evidence that the effect may depend on random impurities in the water.
Fast-freezing of hot water is known as the Mpemba effect, after a Tanzanian schoolboy called Erasto Mpemba (see "How the Mpemba effect got its name"). Physicists have come up with several possible explanations, including faster evaporation reducing the volume of hot water, a layer of frost insulating the cooler water, and differing concentration of solutes. But the answer has been very hard to pin down because the effect is unreliable - cold water is just as likely to freeze faster.
James Brownridge, who is radiation safety officer for the State University of New York at Binghamton, believes that this randomness is crucial. Over the past 10 years he has carried out hundreds of experiments on the Mpemba effect in his spare time, and has evidence that the effect is based on the shifty phenomenon of supercooling.
"Water hardly ever freezes at 0 °C," says Brownridge. "It usually supercools, and only begins freezing at a lower temperature." The freezing point depends on impurities in the water which seed the formation of ice crystals. Typically, water may contain several types of impurity, from dust particles to dissolved salts and bacteria, each of which triggers freezing at a characteristic temperature. The impurity with the highest nucleation temperature determines the temperature at which the water freezes.
Brownridge starts with two samples of water at the same temperature - say, tap water at 20 °C - in covered test tubes and cools them in a freezer. One will freeze first, presumably because its random mix of impurities give it a higher freezing point.
If the difference is large enough, the Mpemba effect will appear. Brownridge selects the sample with the higher natural freezing temperature to heat to 80 °C, warming the other to only room temperature, then puts the test tubes back in the freezer. The hot water will always freeze faster than the cold water if its freezing point is at least 5 °C higher, Brownridge says.
It may seem surprising that moving the finish line by only 5 °C makes enough of a difference, when the hotter sample starts out 60 °C behind in the race. But the bigger the temperature difference between an object and its surroundings - in this case, the freezer - the faster it cools. So the hot sample will do most of its cooling very quickly, helping it to reach its own freezing point of -2 °C, say, before the cooler water gets to its freezing point of -7 °C.
Why has no one else noticed this? Brownridge says other people have not controlled the experimental conditions well enough to study one factor at a time. For instance, it is necessary to control the type of container, the location of a sample in the freezer and so on.
This work is unlikely to end the Mpemba debate. Jonathan Katz of Washington University in St Louis, Missouri, says he is sceptical. According to Katz's own theory, heating raises the freezing point of water by driving off solutes such as carbon dioxide. This means that heating the water actually increases the chances that it will freeze first, unlike the more random outcomes suggested by Brownridge. "Perhaps he has found an effect of supercooling that resembles Mpemba," says Katz.
How the Mpemba effect got its name
This peculiar phenomenon has a long history. It was first remarked on by Aristotle in the 4th century BC. "The fact that water has previously been warmed contributes to its freezing quickly; for so it cools sooner," he wrote. "Hence many people, when they want to cool hot water quickly, begin by putting it in the sun."
The effect was also known to Francis Bacon, who in 1620 wrote, "water slightly warm is more easily frozen than quite cold". Then in 1637 René Descartes said, "Experience shows that water that has been kept for a long time on the fire freezes sooner than other water."
In the 1960s, the effect came to the notice of modern science when a Tanzanian schoolboy called Erasto Mpemba told his science teacher he could make ice cream faster than normal by putting a heated mixture in a freezer. Mpemba was the laughing stock of his class until a school inspector in Dar es Salaam repeated the experiment and vindicated him.
