Tuesday 22 December 2015

Many Years Ago,

I watched a documentary where in one of the Arctic circles, 


 a scientist threw a beaker of nearly boiling water into the air and it froze immediately, he repeated the same thing with cold water and it just fell to the ground unfrozen, the reason the hot water froze is called the Mpemba Effect, knowing this Ontario-based photographer Michael Davies went out temperature of -40° to capture this picture as he sprayed a thermos of hot tea through the air, for the last decade Davies has worked as a photographer in the fly-in community of Pangnirtung in Canada’s High Arctic, only 20km south of the Arctic Circle, a place that sees about two hours of sunlight each day during the winter, so it was some feat to time these pictures, 

and for myself an even better picture with the sun in the centre of the spray, in case you are wondering about name of the phenomenon, it is named after a Tanzanian schoolboy Erasto Mpemba who claimed in his science class that ice cream would freeze faster if it was heated first before being put in the freezer, the laughter ended only when a school inspector tried the experiment himself and vindicated him, that is the short story, here is the long one, 

Erasto Mpemba,

in 1963, Mpemba was making ice cream at school, which he did by mixing boiling milk with sugar, He was supposed to wait for the milk to cool before placing it the refrigerator, but in a rush to get scarce refrigerator space, put his milk in without cooling it, to his surprise, he found that his hot milk froze into ice cream before that of other pupils, he asked his physics teacher for an explanation, but was told that he must have been confused, since his observation was impossible, Mpemba believed his teacher at the time, but later that year he met a friend of his who made and sold ice cream in Tanga town, His friend told Mpemba that when making ice cream, he put the hot liquids in the refrigerator to make them freeze faster, Mpemba found that other ice cream sellers in Tanga had the same practice,
Erasto Mpemba and Denis Osborne (London, 2013).

later, when in high school, Mpemba learned Newton's law of cooling, that describes how hot bodies are supposed to cool (under certain simplifying assumptions), Mpemba asked his teacher why hot milk froze before cold milk when he put them in the freezer, the teacher answered that Mpemba must have been confused, when Mpemba kept arguing, the teacher said "All I can say is that is Mpemba's physics and not the universal physics" and from then on, the teacher and the class would criticize Mpemba's mistakes in mathematics and physics by saying "That is Mpemba's mathematics" or "That is Mpemba's physics." but when Mpemba later tried the experiment with hot and cold water in the biology laboratory of his school, he again found that the hot water froze sooner, earlier, Dr Osborne, a professor of physics, had visited Mpemba's high school, Mpemba had asked him to explain why hot water would freeze before cold water, Dr Osborne said that he could not think of any explanation, but would try the experiment later, when back in his laboratory, he asked a young technician to test Mpemba's claim, the technician later reported that the hot water froze first, and said "But we'll keep on repeating the experiment until we get the right result." however, repeated tests gave the same result, and in 1969 Mpemba and Osborne wrote up their results.

as an aside in the same year, in one of the coincidences so common in science, Dr Kell independently wrote a paper on hot water freezing sooner than cold water, Kell showed that if one assumed that the water cooled primarily by evaporation, and maintained a uniform temperature, the hot water would lose enough mass to freeze first, Kell thus argued that the phenomenon (then a common urban legend in Canada) was real and could be explained by evaporation, however, he was unaware of Osborne's experiments, which had measured the mass lost to evaporation and found it insufficient to explain the effect, subsequent experiments were done with water in a closed container, eliminating the effects of evaporation, and still found that the hot water froze first,

but how strange that a schoolboy in Africa should have a ice forming experiment in the Arctic circle named after him.


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