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The Tale of the Mpemba Effect
« on: 2002-08-24 16:13:53 » |
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The Tale of the Mpemba Effect
Tanzania, 1963. A high-school boy, Mpemba, was with a friend who made and sold ice cream in Tanga town. Mpemba noticed that his friend was hurrying to put the ice cream in the fridge while still hot. He asked why, and his friend explained that this way the ice cream would freeze faster, a well known trick of the trade among ice-cream makers.
Mpemba tried it himself at home with two pots of hot and cool water and found that it was true. The hot water froze first. Next day at school, he asked his physics teacher:
"Sir, why does hot water freeze faster than cool water?".
"Don't be a fool!" his teacher replied, "That is absurd! Look here! When the water is at a temperature of 40 C it needs an amount of time to go to 0 C. When the water is at 80 C it needs first an amount of time to go to 40 C and then another amount of time to go to 0 C."
"But the hot water did freeze first!" said Mpemba.
"Well, kids", said the teacher to everyone's amusement, "Remember that! This is Mpemba Physics!"
The next day in a Math class, someone made a silly mistake and the class roared "Hey, this is Mpemba Mathematics!"
After a while, Dr. Osborne, a university professor, came by Mpemba's school for a lecture on physics. When he asked if there were any questions, Mpemba stood up, to the dismay of his teacher and to the amusement of his classmates.
"Sir, why does hot water freeze faster than cool water?".
"Well", Dr. Osborne said, "I am not sure you got it right, but I can promise I'll look into it and answer you at another time."
Back at the university, Dr. Osborne told a lab assistant to do the experiment and take measurements of the freezing time with pots of water at different temperatures. A couple of days later, he remembered to ask for the measurements.
"I am so sorry Professor", the assistant said. "We have been repeating the experiment all this time but the results we are getting are all wrong! Something is happening and the hot water freezes first! But we'll keep trying until we get the right results."
Some years later, a paper was published:
Mpemba and Osborne, Phys. Educ., "Cool," v.4, p.172-5 (1969)
The Mpemba Effect has been studied a lot since that time. It has been attributed to a combination of several different physical phenomena involving evaporation, dissolved gasses, convection, and the surroundings, which may or may not occure during freezing, none of them clearly dominant. So, it has become clear that the process of freezing cannot be explained just by the temperature.
However, you can still find a lot of web pages classifying this effect or some specific way to achieve it as a cultural legend.
It is interesting to note that the Mpemba Effect was actually a rediscovery, not a new discovery. Except for the ice-cream makers in Tanzania and all over the world, it had been noticed by:
1. Aristotle in "Meteorologica I" (circa 300 BC)
"The fact that water has previously been warmed contributes to its freezing quickly; for so it cools sooner. Hence many people, when they want to cool hot water quickly, begin by putting it in the sun. . ."
Aristotle wrote that in support of his theory of "antiperistasis", which was a supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality, for instance, the sudden heating of a warm body when surrounded by cold.
2. Giovanni Marliani (1461) reported that he had experimentally confirmed the effect, but was unable to give an explanation.
3. Bacon (1620): "Water slightly warm is more easily frozen than quite cold"
4. Descartes: "Experience shows that water which has been kept for a long time on the fire freezes sooner than other water".
With Newton's law of cooling, around 1700, and the subsequent theory of heat, which were satisfactory tools for just about everything else, this strange effect was conveniently forgotten by the scientific community and honored only by some laymen such as ice-cream makers until its rediscovery in 1969.
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