The Mpemba Effect


For all that we know about the physical world, there are a few phenomena that, though seemingly simple, continue to baffle us.

And one of the most baffling is the Mpemba Effect.

You may not know it by that name—I didn’t until I read an article on New Scientist’s website last week—but you’ve probably heard about it. Heck, you may even have gotten in an argument about it.

The Mpemba Effect is the proper name of the counterintuitive fact that sometimes hot water freezes faster than cold water.

As New Scientist explains, Aristotle remarked on this phenomenon in the 4th century B.C., and Francis Bacon wrote about it in 1620. Just 17 years after that, in the very same year in which he famously wrote “Cogito ergo sum” (“I think, therefore I am”), René Descartes took time out from philosophical musings to note that, “Experience shows that water that has been kept for a long time on the fire freezes sooner than other water.”

Still, the phenomenon didn’t even have a name until the 1960s, when Erasto Mpemba, a Tanzanian schoolboy, told his science teacher that he could make ice cream faster than usual by putting a heated mixture in a freezer. His classmates laughed at him, but a school inspector in Dar es Salaam successfully repeated the experiment.

Why should something that’s hot to start with freeze faster than something that’s already cold? There have been numerous attempts at explanations, but they’re frustrated by the fact that the effect is unreliable. While hot water will, indeed, sometimes freeze faster than cold water, it doesn’t always freeze faster than cold water. In fact, cold water is just as likely to freeze first.

Over the past 10 years, James Brownridge, radiation safety officer for the State University of New York at Binghamton, has carried out hundreds of experiments on the Mpemba effect (in his spare time, not as part of his duties), and he believes he has the answer.

According to Brownridge, the effect is based on another phenomenon called supercooling, the fact that even though we all know zero degrees Celsius is the freezing point of water, water hardly ever freezes right at zero degrees. Usually it cools below that point before it begins to freeze.

The actual freezing point depends on impurities in the water. These impurities—which can range from dust particles to bacteria to dissolved salts—serve as “seeds” for the formation of ice crystals, and different types of impurities trigger freezing at different temperatures. The impurity that causes freezing at the highest temperature determines when the water will start to freeze.

In his experiments, Brownridge starts with two samples of water at the same temperature in covered test tubes. He cools them in a freezer. One will freeze first, presumably because of its random mix of impurities. Brownridge then takes the sample with the higher natural freezing temperature and heats it to 80 degrees Celsius, warming the other only to room temperature. Then he puts them back in the freezer. If, in the initial test, the hot-water sample’s freezing temperature was at least five degrees Celsius higher than the other sample, it will always freeze first.

Why does that five degrees make such a difference when the hot-water sample has a full 60 degrees of extra temperature to shed compared to the room-temperature sample? Because the higher the temperature difference between an object and its surroundings, the faster it cools. That means the hot sample sheds all of its extra heat and reaches its five-degrees-higher freezing point of, say, minus-two, before the room-temperature sample, cooling more slowly, reaches its lower freezing point of, say, minus-seven.

Is it a matter of case solved, then? Not hardly. As New Scientist points out, Jonathan Katz of Washington University in St. Louis, for one, doesn’t buy it. In his theory, heating raises the freezing point of water by driving off solutes such as carbon dioxide, which means heating water increases the chance that it will freeze first, rather than it being purely random as Brownridge suggests.

It may all seem rather unimportant, as scientific debates go, but it’s a fascinating reminder that the world is a very complex place about which there’s always more to discover…even in our own kitchens.

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