Mpemba Effect

Sam Cates

In 1963, Erasto Mpemba was freezing ice cream mix, during a school experiment in Tanzania; and, seeming to defy common sense, he noticed that warm mix solidified far faster than colder mix when placed in a freezer. Though he was understandably ridiculed by his teacher and fellow students, Erasto had just observed one of the most counterintuitive, baffling and controversial phenomena in fluid dynamics, later named The Mpemba Effect.

What he had discovered, in principle, was that hot water freezes faster when placed in sub-zero temperatures. Now, the reason why this is controversial is that many physicists dispute the conditions needed for it to occur; and nobody at all can be sure why it happens.

At a glance, the statement seems immediately wrong. Hot fluid must lose more energy in heat, and at some stage will be the same temperature as the colder fluid was at the beginning. This is all true, but ignores the fact that the water doesn’t remain at a uniform temperature, as liquid near the centre is insulated by the substances around it.

Water is not a fantastic insulator; if it were, radiators would be somewhat pointless. But its colder cousin - ice - does a far better job, which is why igloos are not a terrible way to stay warm in the snow. And this is the first difference between the two temperatures; because it is cooler and closer to freezing, a layer of frost forms on the cold water quicker than its warm counterpart, and insulates the water in the centre far more effectively. If you have ever impatiently watched an ice cube freeze, you will know that its centre is always the last to solidify. So, because more heat from the centre of hot water can escape before a layer of frost forms, the time taken to freeze the entire liquid is reduced.

Sadly though, this alone isn’t enough to account for the wide time difference, and many believe that other factors play a far greater role. Steam given off, for example, can slightly reduce the volume of hotter water. The strength of hydrogen bonds holding the liquid together are peculiarly distributed, and results from vibrational spectroscopy show the number of stronger bonds increase as the temperature rises, meaning freezing the entire liquid takes more time but less energy. Other theories are even more complex: supercooling (lowering the temperature of a liquid below its melting point without it becoming a solid) and deviations from the Maxwell-Boltzmann distribution to describe particle velocity have both been put forward as explanations. Statistical Mechanics has also predicted the existence of the Inverse Mpemba Effect, where heating a cooled species takes less time than a warmer one, closer to equilibrium.

Science and technology have never been changing as fast than they are now. However, for all our knowledge of quantum mechanics, artificial intelligence, and astronomy, it is interesting to note that 8 billion minds have no explanation for something as simple as freezing water.

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