The Mpemba Effect is a term for possible cases in which hot water freezes faster than cold water. It does not always happen. If the water is at first hot, cold water at the bottom is denser than the hot water at the top, so no convection will occur and the lower part will start freezing while the top is still warm. This effect, combined with evaporation, may make hot water freeze faster than cold water in some cases.
It is named after a thirteen-year-old Tanzanian schoolboy who boiled some milk, added sugar and kept the hot mixture in the refrigerator. An hour and a half later when he went back with another boy, he found that his mixture had frozen into ice cream, while the other boy’s mixture (which was kept inside at room temperature) was still not frozen. A bit of the mixture froze faster than a cold mixture!
The boy, Mpemba, confronted his teacher about this, only to be rejected and ridiculed. He was told that his observation belonged to “Mpemba’s physics and not the real universal physics”. But, the people who ridiculed him refused to ‘look into the telescope’ and try observing, much like the hierarchy back in Galileo’s time.
With the help of Dr Osborne (not the Spiderman villain), a professor, Mpemba recreated the effect by placing 100 ml breakers holding 70 ml of water at various temperatures and measured the time taken to freeze. They found that the water at 25° C took the longest time, and water at 90° C took the least.
Turns out that many historical texts, such as those of Aristotle, Francis Bacon and Rene Descartes were also found to bear descriptions of hot water freezing faster than cold water.
Avinash Kumar and John Bechhoefer in recent experiments showed that the Mpemba effect is real, isn't specific to water and isn’t specific to freezing. This means that a wide range of substances would exhibit the Mpemba effect and that there are two objects at temperatures T1 and T2 ( where T1 >T2) the first object will reach a temperature T3 (where T3< T2 <T1) faster.
As of now, there are many explanations for the effect but none are widely accepted. Mpemba’s story serves as a reminder to us of the importance of experiment and observation in the scientific enterprise.
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