The Mpemba Effect, or why hot water freezes faster than cold?
The Mpemba Effect (Mpemba Paradox) is a paradox that suggests under certain conditions, hot water freezes faster than cold water, even though it should pass through the temperature of cold water during the freezing process. This paradox is an experimental fact that contradicts traditional beliefs, which state that under the same conditions, it takes longer for a more heated body to cool to a certain temperature than for a less heated body to cool to the same temperature.
This phenomenon was noted in the past by Aristotle, Francis Bacon, and René Descartes, but it was only in 1963 that Tanzanian schoolboy Erasto Mpemba established that a hot ice cream mixture freezes faster than cold.
While attending Magamba Secondary School in Tanzania, Erasto Mpemba conducted a practical assignment on cooking. He needed to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then refrigerate it to freeze. Perhaps not being particularly diligent, Mpemba started with the first part of the task. Afraid he wouldn't finish by the end of the lesson, he put the hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his classmates, prepared according to the prescribed technology.
After this, Mpemba experimented not only with milk but also with plain water. In any case, while still a student at Mkwawa Secondary School, he asked Professor Dennis Osborne from the University College in Dar es Salaam (who was invited by the school principal to lecture the students on physics) about water: "If you take two identical containers with equal volumes of water, with one having a temperature of 35°C and the other 100°C, and put them in the freezer, the water in the second container will freeze faster. Why?" Osborne became interested in this question, and soon in 1969, they together with Mpemba published the results of their experiments in the journal "Physics Education." Since then, the effect they discovered has been called the Mpemba Effect.
Until now, no one knows exactly how to explain this strange effect. Scientists do not have a single version, although there are many. It all comes down to the difference in properties between hot and cold water, but it is still not clear which properties play a role in this case: the difference in supercooling, evaporation, ice formation, convection, or the influence of rarefied gases on water at different temperatures.
The paradox of the Mpemba Effect is that the time it takes for a body to cool to the temperature of the surrounding environment must be proportional to the difference in temperatures between that body and the surrounding environment. This law was established by Newton long ago and has since been confirmed many times in practice. However, in this effect, water at 100°C cools to 0°C faster than the same amount of water at 35°C.
