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 freezemore 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. Butwhat is behind the so-called "Mpemba effect"?

According to Jonathan Katz of the Washington University in 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 itprecipitates 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 akettle.

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.

There is a second, related effect that hampers the freezing of water that has never been heated. The lowering of the freezing point reduces the temperature difference between the liquid and its freezing surroundings. "Since the rate at which heat is lost from the water depends on this
temperature difference, water that has not been heated has greater difficulty losing heat," Katz says.

Katz claims that the two effects combined can perfectly explain why water that has been heated freezes more quickly than water that hasn't. And he makes a prediction that experiments should be able to verify: that the Mpemba effect should be more marked the "harder" the water. "This may explain why not everyone sees it," he says. "Some people are using soft water." "Katz's analysis of the Mpemba effect is deeper and more rigorous than anything else on the subject," says Richard Muller of the University of California at Berkeley. "He has come up with a simple yet I believecorrect way to look at a complex phenomenon."