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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." |
