Why water freezes faster after heating

[XMEN Iceman]

I always wondered about this...finally found the answer. - Ice

http://www.eurekalert.org/pub_releases/ ... 053106.php

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 ¨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"?

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 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.

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 believe correct way to look at a complex phenomenon."

Katz, who worked out the details of the Mpemba effect while adjudicating a student exam, is waiting for someone to do the experiment to test his theory. "It's not difficult but it's not trivial either," he says. "I think it would take a couple of months to do it right."


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[XMEN Ashaman DTM]

Actually, the effect is explained quite nicely with thermodynamics.

With hot water, there are more fast moving molecules. And a lot of the ones near the surface of the water move fast enough to jump out (called "steam" by some people). So the hot water loses heat more quickly (having a surplus of fast moving molecules to carry more heat away). What you get is water whose temperature drops very fast, compared to the cooler water.

The hot water has less water when it finally freezes though, if you were to start out with equal amounts of water.

The thing the article mentions has been known for many decades. The impurities in water both act as binding sites for ice crystals, and as a binding site for steam bubbles to form when the water boils. Cracks and deformities on an atomic scale on the container the water is sitting in do the same thing. It's ALSO the same process that allows the whole semiconductor industry to exist: chips are nothing more than silicon with certain impurities added on purpose to ease or restrict the flow of electrons.





It's interesting to note how some of the sciences haven't caught up with each other. You see it quite often in some of the work that I do. All of my above knowledge came from my studies of fluid mechanics.

[TimberWolf]

I learned about this stuff in my Material Science and Engineering this quarter. I will be taking Thermodynamics next year to get more indepth look at how heat effects engines and mechanical components.