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Mpemba effect
The Mpemba Effect, named after Erasto Mpemba, is the observation that, in some circumstances, warmer water can freeze faster than colder water. Although there is evidence of the effect, there is disagreement on exactly what the effect is and under what circumstances it occurs. There have been reports of similar phenomena since ancient times, although with insufficient detail for the claims to be replicated. A number of possible explanations for the effect have been proposed. Further investigations will need to decide on a precise definition of "freezing" and control a vast number of starting parameters in order to confirm or explain the effect.
== Definition ==
The phenomenon, when taken to mean "hot water freezes faster than cold", is difficult to reproduce or confirm, because this statement is ill defined.〔 Jeng proposes as a more precise wording:
There exists a set of initial parameters, and a pair of temperatures, such that given two bodies of water identical in these parameters, and differing only in initial uniform temperatures, the hot one will freeze sooner.
However, even with this 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 .〔 A quantity of water can be at and not be ice; after enough calories (i.e., heat) have been removed to reach more calories must be removed before the water changes to solid state (ice), so water can be liquid or solid at .〔High School Chemistry and Physics〕
With the above definition there are simple ways in which the effect might be observed: For example if the hotter temperature melts the frost on a cooling surface and thus increases the thermal conductivity between the cooling surface and the water container.〔 On the other hand there may be many circumstances in which the effect is not observed.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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