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Natural phenomenon that hot water freezes faster than cold

The Mpemba effect is the name given to the observation that a liquid (typically water) which is initially hot can freeze faster than the same liquid which begins cold, under otherwise similar conditions. There is disagreement about its theoretical basis and the parameters required to produce the effect. The Mpemba effect is named after Tanzanian scientist Erasto Bartholomeo Mpemba, who described it in 1963 as a secondary school student. The initial discovery and observations of the effect originate in ancient times; Aristotle said that it was common knowledge.

The phenomenon, when taken to mean "hot water freezes faster than cold", is difficult to reproduce or confirm because it is ill defined. Even with Jeng's definition, it is not clear whether "freezing" refers to the point at which water forms a visible surface layer of ice, the point at which the entire volume of water becomes a solid block of ice, or when the water reaches Aristotle's explanation involved antiperistasis: " the supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality." Francis Bacon noted that "slightly tepid water freezes more easily than that which is utterly cold." René Descartes wrote in his Discourse on the Method, relating the phenomenon to his vortex theory: "One can see by experience that water that has been kept on a fire for a long time freezes faster than other, the reason being that those of its particles that are least able to stop bending evaporate while the water is being heated." Scottish scientist Joseph Black investigated a special case of the phenomenon by comparing previously boiled with unboiled water; he found that the previously boiled water froze more quickly. Evaporation was controlled for. He discussed the influence of stirring on the results of the experiment, noting that stirring the unboiled water led to it freezing at the same time as the previously boiled water, and also noted that stirring the very cold unboiled water led to immediate freezing. Joseph Black then discussed Daniel Gabriel Fahrenheit's description of supercooling of water, arguing that the previously boiled water could not be as readily supercooled.

The effect is named after Tanzanian scientist Erasto Mpemba. He described it in 1963 in Form 3 of Magamba Secondary School, Tanganyika; when freezing a hot ice cream mix in a cookery class, he noticed that it froze before a cold mix. He later became a student at Mkwawa Secondary (formerly High) School in Iringa. The headmaster invited Dr. Denis Osborne from the University College in Dar es Salaam to give a lecture on physics. After the lecture, Mpemba asked him, "If you take two similar containers with equal volumes of water, one at and the other at , and put them into a freezer, the one that started at freezes first. Why?" Mpemba was at first ridiculed by both his classmates and his teacher. After initial consternation, however, Osborne experimented on the issue back at his workplace and confirmed Mpemba's finding. They published the results together in 1969, while Mpemba was studying at the College of African Wildlife Management. Mpemba and Osborne described placing samples of water in beakers in the icebox of a domestic refrigerator on a sheet of polystyrene foam. They showed the time for freezing to start was longest with an initial temperature of and that it was much less at around They ruled out loss of liquid volume by evaporation and the effect of dissolved air as significant factors. In their setup, most heat loss was found to be from the liquid surface. In 2016, Burridge and Linden defined the criterion as the time to reach , carried out experiments, and reviewed published work to date. They noted that the large difference originally claimed had not been replicated, and that studies showing a small effect could be influenced by variations in the positioning of thermometers: "We conclude, somewhat sadly, that there is no evidence to support meaningful observations of the Mpemba effect." Experimental results confirming the Mpemba effect have been criticized for being flawed, not accounting for dissolved solids and gasses, and other confounding factors. Philip Ball, a reviewer for Physics World wrote: "Even if the Mpemba effect is real — if hot water can sometimes freeze more quickly than cold — it is not clear whether the explanation would be trivial or illuminating." New Scientist recommends starting the experiment with containers at , respectively, to maximize the effect. In a related study, it was found that freezer temperature also affects the probability of observing the Mpemba phenomenon as well as container temperature does.