Mpemba effect

Episode #7 of the course “Brain-twisting paradoxes”

The Mpemba effect is the phenomenon of hot water freezing faster than cold water. This occurrence was discovered in 1969 by a high school student named Erasto Mpemba in Tanzania. Due to the extremely counterintuitive nature of this finding, most scientists were surprised by Mpemba’s discovery. Only after several years of experimentation did the scientific community at large accept his discovery.

While the effect is real, it does not happen under all conditions, and debates still occur as to the exact reason and conditions under which it takes place. Theories to explain why it happens have included:

• Hot water evaporates faster than cold water, thereby reducing the volume and thus time left to freeze.

• A frost layer forms on cold water, thereby insulating it.

• The heated water drives off solutes such as carbon dioxide, thereby changing the rate of freezing.

The simple statement of hot water freezing faster than cold water certainly needs some parameters. A situation in which the Mpemba effect does not occur, for example, is when the hot water starts at 99.9°C, and the cold water at 0.01°C. The difference in temperature is simply too much for the effect to overcome. The same concept applies to the shape of the containers as well as the cooling conditions.

Although Mpemba is credited with the discovery in modern times, a number of philosophers and scientists from the past have noted the effects of heat on freezing water as well. Aristotle in the 4th century BC, then Francis Bacon and René Descartes have all written about it on various occasions. For a question that has been around for so long, it is certainly a mystery how modern science has not answered this seemingly simple question about cooling water.

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