Kyre Austin (Boston) THIS ARTICLE APPEARS IN NEW SCIENTIST
MAGAZINE ISSUE: 3 JUNE 2006 AUTHOR: MARCUS CHOWN A common
chemical process may explain a bizarre property of water that
has been a mystery since the time of Aristotle ĘC 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"? Jonathan Katz of 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 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