back MAKING ICE CREAM the Aristotle way next
Here is a story that reminds us not to make
quick judgements about what is possible or impossible.
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 students. 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 [5].
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 [11]. 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 [14].
It is still not known exactly why this happens. A number of possible explanations for the effect have been proposed, but so far the experiments do not show clearly which, if any, of the proposed mechanisms is the most important one.
Modified for private educational use only. Original Source Copyright
Written Nov, 1998 by Monwhea Jeng (Momo),
Department of Physics, University of California