Posted by: Prospector PJ1 AUG 2012
If you put a container of hot water and a container of cold water in a freezer, which freezes first? Although no one knows exactly why, it is the hot water.
This so-called Mpemba effect has baffled some of the world’s greatest thinkers, from Aristotle to Francis Bacon, Descartes and the smartest physicists of the 21st century. Although various explanations have been suggested, no one has come up with a definitive solution.
The phenomenon is named after Tanzanian schoolboy Erasto Mpemba, who noticed it during a cookery class in 1963, when he observed that a hot ice cream mix froze before a cold mix. When he asked a visiting physics lecturer to explain the effect Mpemba was ridiculed by his teacher and classmates. But on confirming the observation back at his university, the physicist invited Mpemba to be joint author of paper on the matter in 1968.
Possible explanations include evaporation of warmer water reducing the mass of water to be frozen, plus the endothermic effect of evaporation cooling the remaining water. Higher convection in the warmer water may spread ice crystals around faster, and the hotter container may melt through an insulating layer of frost, bringing it into contact with a much colder surface beneath. Dissolved solutes could also play a part.
The Royal Society of Chemistry has offered a £1,000 prize for the person or team who can produce the “best and most creative” explanation of the phenomenon. The public had until the end of July to come up with ideas before some of the world’s brightest young scientists tackle the puzzle at a science meeting in London.