Science & Technology

Why Can Hot Water Freeze Faster Than Cold Water?

Under certain conditions, hot water freezes faster than cold water. It goes against everything you might assume, but it's true — minds as great as Aristotle, Descartes, and Francis Bacon have mentioned it happening. But it wasn't officially described in a science journal until 1969, thanks to a partnership between a 13-year-old Tanzanian student named Erasto B. Mpemba and a traveling physics professor named Dr. Denis Osborne.

The Story of the Mpemba Effect

The Mpemba effect, as it is now called, started with ice cream. Because Mpemba was in a rush to get his ice cream in the freezer before other students had taken up all the space, he decided to put his boiling milk in without letting it cool first. Surprisingly, he found that his milk froze long before anyone else's. Mpemba asked his teachers why this might be, and most told him he must be mistaken — except for physics professor Dr. Denis Osborne. He gave the question some thought and asked a lab technician to test Mpemba's claim.

Sure enough, experiments showed that under certain conditions — conditions scientists still don't exactly agree on — hot water freezes faster than cool water. In 1969, Osborne and Mpemba published a paper about the phenomenon. The fact that the effect doesn't happen every time makes it difficult to study, but there is no shortage of proposed physical mechanisms at play.

Related Video: What the Heck Is the Mpemba Effect?

What Causes the Mpemba Effect?

For a long time, the most popular hypothesis was that hot water evaporates more quickly, so it loses more mass and needs to lose less heat to freeze. But scientists have observed the Mpemba effect in closed containers with no evaporation. Supercooling, the idea that dissolved gases in the water might speed the freezing process, is also a possibility.

In 2013, the media claimed that a team of researchers from Singapore had found the answer: The bonds between molecules in boiling water are more flexible and ready to give up energy (in the form of heat) than those in cool water. Unfortunately, scientists don't find their explanation as airtight as the public did. The problem with most of the theories proposed thus far is that they don't explain why the effect only happens sometimes.

The latest hypothesis from January of this year considers the effect of entropy on water and relies on the idea that the bonds between H2O molecules structurally change in response to temperature. Entropy describes the amount of disorder in a system, and hot water, with its constantly moving molecules, has more entropy than cold water. In fact, cold water is "sticky," molecularly speaking — the molecules like to cluster together and make weak bonds with each other's hydrogen atoms. But as the water heats up, these bonds break and the clusters turn into disordered swarms of molecules. Just as you'll get colder on your own than when you're huddled with a group of friends, these buzzing hot-water molecules may be able to form ice crystals in a freezer faster than the clusters of cold-water molecules, since they have a tighter arrangement that takes time for the temperature to change.

Importantly, this effect wouldn't necessarily appear if there were a lot of salts and minerals dissolved in the water, since that water wouldn't form as many clusters at cold temperatures. This explanation neatly addresses both the phenomenon itself and the difficulty of repeating it under laboratory conditions — but that doesn't necessarily mean it's true, either.

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For more explanations for wild science phenomena, check out "Ask a Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works" by Larry Scheckel. The audiobook is free with an Audible trial. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Austin Jesse Mitchell September 29, 2016

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