Hot Water Can Freeze Faster Than Cold Water, And Nobody Knows Why

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 described it happening, but it became an experimentally proven fact in 1969, thanks to 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 – starts with ice cream. Because he was in a rush to get his ice cream in the freezer before other students had taken up all the space, Mpemba 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. But Dr. Denis Osborne thought about his question 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 which ones – hot water cools faster than cool water. In 1969, Osborne and Mpemba published a paper about the phenomenon. The fact that effect doesn't happen every time makes it difficult to study, but there is no shortage of proposed physical mechanisms at play.

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 does. The problem with most of the theories proposed thus far is that they don't explain why the effect only happens sometimes.

The very latest hypothesis considers the effect of entropy on warmer bodies of 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. If hydrogen-to-hydrogen bonds break in warm water, leading to increased entropy, then the individual molecules may be able to form ice crystals faster than if the change in temperature has to travel along an already-existing network of weak hydrogen bonds. Essentially, the change in temperature has to "travel" along a network that is more complex in colder water than in warmer water. 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.

Does Hot Water Freeze Faster Than Cold Water?

The answer? Yes—sometimes.

Written by Austin Jesse Mitchell September 29, 2016

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