The Mpemba effect refers to the curious observation that hot water can sometimes freeze faster than cold water. This phenomenon was first reported by Erasto Mpemba, a high school student in Tanzania, in 1969.
Erasto Mpemba in his early years
Mpemba discovered this effect while making ice cream. He noticed that when he used hot milk, it froze faster than when he used cold milk. He shared this observation with his physics teacher, Dr. Denis G. Osborne, who initially thought it was impossible. However, after conducting several experiments, Osborne confirmed that Mpemba was correct.
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 Mpemba 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. A situation in which the Mpemba effect does not occur, for example, is when the hot water starts at 99.9°C (211.8°F), and the cold water at 0.01°C (32°F). 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.
Scientists have been trying to explain the Mpemba effect ever since it was discovered. There are several proposed explanations for why hot water may freeze faster than cold water. One theory suggests that hot water may evaporate more quickly than cold water, which could reduce its volume and allow it to freeze more rapidly. Another theory suggests that a layer of frost may form on cold water, insulating it and slowing down the freezing process. A third explanation is that heating the water could cause it to release dissolved gasses, such as carbon dioxide, which can affect the rate of freezing. Despite these proposed explanations, the Mpemba effect remains a topic of ongoing research and debate among scientists.
John Bechhoefer and Avinash Kumar, both physicists at Simon Fraser University, published a study in 2020 that provides further evidence supporting the Mpemba effect. They chose to concentrate their research efforts on microscopic glass beads instead of freezing water, which can be unexpectedly complex to study. By observing how the glass beads moved in water under precise conditions, they discovered that in certain situations, the beads that were initially very hot cooled more rapidly than those that weren't. They repeated the test over 1,000 times, dropping the beads in different wells and starting at different temperatures.
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 still has not answered this seemingly simple question about cooling water.
“Nothing is softer or more flexible than water, yet nothing can resist it.” —Lao Tzu
“If there is magic on this planet, it is contained in water.” —Loren Eiseley
“It is life, I think, to watch the water. A man can learn so many things.” —Nicholas Sparks
“Human nature is like water. It takes the shape of its container.” —Wallace Stevens
Dylan Ris. The Mpemba Effect: Does Hot Water Really Freeze Faster Than Cold Water? HowStuffWorks, 2023.
Monwhea Jeng. Can hot water freeze faster than cold water? Department of Physics, University of California, 1988.
Theresa Machemer. The Physics of Why Hot Water Sometimes Freezes Faster Than Cold Water. Smithsonian Magazine, 2020.
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