The Mpemba Effect

 

Hot and cold water-The Mpemba effect

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Hot Water Freezes Faster Than Cold Water

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

The Mpemba effect is named after Tanzanian schoolboy Erasto Bartholomeo Mpemba (1950–2020) whose story of it in 1963 became highly popularized. The discovery noted its observations, however, originate in ancient times, having been said by Aristotle to be common knowledge.

The phenomenon, when taken to mean "hot water freezes faster than cold", is difficult to reproduce or confirm because this statement is ill-defined. Though a more precise wording was made by Monwhea Jeng, a  Principal Applied Scientist:

"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."

 

Even with this definition it is not clear, however, 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 0 °C (32 °F; 273 K).

Several effects of heat on the freezing of water were described by ancient scientists such as Aristotle:

 

"The fact that the water has previously been warmed contributes to its freezing quickly: for so it cools sooner. Hence many people, when they want to cool water quickly, begin by putting it in the sun. So the inhabitants of Pontus when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their reeds that it may freeze the quicker, for they use the ice like lead to fix the reeds."

Early modern scientists such as Francis Bacon noted: 

"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."

Tanzanian Erasto Mpemba, whom the effect was named after, described it in 1963 in Form 3 of Magamba Secondary School, Tanganyika, when freezing ice cream mix that was hot in cookery classes and noticing that it froze before the 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 the central question:

"If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) 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 70 ml (2.5 imp fl oz; 2.4 US fl oz) samples of water in 100 ml (3.5 imp fl oz; 3.4 US fl oz) 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 25 °C (77 °F) and that it was much less at around 90 °C (194 °F). They ruled out loss of liquid volume by evaporation as a significant factor and the effect of dissolved air. In their setup, most heat loss was found to be from the liquid surface.

Credit _ Wikipedia