It’s called the Mpemba Effect, and it is a mystery that has intrigued the greatest minds in history for over 2,000 years. As counter-intuitive as it may seem, hot water seems to freeze faster than cold water. Is this true, and if so, how can it be?
Since the time of Aristotle, people have pointed out that hot water has a faster freezing time. Not only did Aristotle ponder this mystery, but so did Rene Descartes and Francis Bacon. When a teenager from Tanzania noticed that heated ice-cream mixtures freeze more rapidly than their chilled counterparts, he was surprised that no one had formally studied the phenomenon. This teenager was Erasto Mpemba, When Dr. Denis G. Osborne from the University College in Dar es Salaam visited Mpemba’s school, the boy asked him, “If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first. Why?”
Osborne was stumped. He tried the experiment at home and confirmed the boy’s observation. Working with Mpemba, the two began laboratory studies to confirm the phenomenon under controlled conditions. The result was official confirmation of the effect, but they were unable to come up with an explanation for why it occurred.
It wasn’t until 2017 when a team of Spanish physicists figured out how and why this apparent paradox can occur. The answer was published in a paper in Physical Review Letters by Antonio Lasanta of Charles III University Madrid and his colleagues. They concluded that it all depends on the speed of individual water particles.
The researchers started by imagining two beakers of water, one hotter and one colder, that are placed in a freezer. Inside each beaker, the molecules that make up the water are swarming in all directions. If the water warms up, they move faster; if it cools down they slow to a crawl; and if it freezes they get stuck in place.
Logically, the time it takes for each beaker of water to freeze depends only on its initial temperature. Particles in the hotter water move faster, which means they have more slowing to do, so the hotter the liquid, the longer it should take.
The researchers discovered something that hadn’t been factored into the equations before. Instead of focusing solely on the average speed of all of the molecules, they tried to account for the outliers. Some molecules jet around substantially faster than average, while others drag their feet, crawling at a markedly slower pace. This degree of deviation from the average, a property known to statisticians as kurtosis, had been neglected in earlier studies.
When conditions are just right – if the hot beaker is the right amount hotter than the cold beaker and its molecules are wayward enough to generate high kurtosis – the model simulation shows that the hotter sample will cool faster than the colder one.
“In fact,” says Lasanta, “we find not only that the hottest can cool faster but also the opposite effect: the coldest can heat faster, which would be called the inverse Mpemba effect.”
Perhaps once this phenomenon is fully explained, researchers will be able to focus their attention on answering the age-old question: “Does watching a pot cause its contents to boil more slowly?”
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