Why water freezes faster after heating

THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 3 JUNE 2006

A common chemical process may explain a bizarre property of water that has been a mystery since the time of Aristotle ¨C how hot water can freeze more quickly than cold.

This strange and counterintuitive effect was first observed by the ancient Greek philosopher and was made famous in recent times by a Tanzanian school student called Erasto Mpemba. He noticed that the sugared milk he was using to make ice cream froze more quickly if it started out hot. But what is behind the so-called "Mpemba effect"?

According to Jonathan Katz of the University of Washington at St Louis, it's all to do with solutes. "You have to ask yourself: what does heating do to water that makes it easier to freeze?" he says. "The answer is that it precipitates out solutes."

The solutes Katz has in mind are calcium and magnesium bicarbonate, which make most drinking water "hard". When the water is heated, these precipitate out to form the solid scale that "furs" up the inside of a kettle.

Water that has never been heated still contains these solutes. As it freezes, ice crystals form, and the concentration of solutes in the remaining water becomes ever higher up to 50 times as high as normal. This lowers the freezing point of the water, just like salt sprinkled on a road in winter. "The water therefore has to cool further before it freezes," says Katz. [...]

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