The Mpemba effect is the somewhat surprising phenomenon whereby hot water can, under certain conditions, freeze faster than cold.
The effect was discovered by the Tanzanian high-school student Mpemba in 1967 after observing the freezing of ice cream in cookery classes. At first sight, the effect is contrary to Newton's law of cooling. However, it has been widely reproduced though it is still poorly understood. The effect is not universal under all experimental conditions so its exact requirements have proved difficult to specify.
It is believed that the effect arises from some interaction between:
- Evaporation reducing the volume to be frozen;
- Effect of boiling on dissolved gases;
- Convection;
- Effects from surroundings such as the insulating effects of frost; and
- Supercooling.
The effect was known to ancient and medieval scientists such as Aristotle, Francis Bacon and René Descartes, but modern science had reduced it to folklore.
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