Wednesday 6 June 2018


The following true story is a cautionary tale for scientists against making quick judgements about what seems impossible. Or indeed for any "experts" believing that they know best and ignoring any evidence contrary to what they believe. Does this ring any bells relating to the global warming hypothesis?

In 1963, aTanzanian schoolboy called 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.

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 .

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.

Subsequent discussion of the effect has been inconclusive.  While quite a few experiments have replicated the effect, there has been no consensus on what causes the effect.  The different possible explanations are discussed .  The effect has repeatedly a topic of heated discussion in the "New Scientist", a popular science magazine.  The letters have revealed that the effect was known by laypeople around the world long before 1969.  Today, there is still no well-agreed explanation of the Mpemba effect.

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