THE MPEMBA EFFECT

Tanzania, ice cream and international scientific controversy wouldn’t win many points in a word association game. But add the name ‘Mpemba’ and there is a link. ‘Tanzanian Affairs’ has uncovered a mystery that began in ancient Greece, resurfaced in 1960’s Tanga and has since ricocheted around university labs the world over.

And we exclusively trace the Tanzanian boy at the centre of the puzzle. His name now fills reruns of scientific journals but at the time he was mocked by his classmates as the ‘black sheep’ of the class.

Erasto Mpemba was the ‘black sheep’. He takes up the story in ‘Physics Education’. “In 1963 when I was in Form 3 in Magamba Secondary School, Tanzania I used to make ice cream. The boys at the school do this by boiling milk, mixing it with sugar and putting it into the freezing chamber in the refrigerator …. a lot of boys make it and there is a rush to get space”. One day Mpemba began boiling milk but another boy dashed to grab the space. Rather than miss his chance Mpemba acted rashly. “I decided to risk ruin to the refrigerator on that day by putting hot milk into it”. His gamble led to a strange discovery.

“The other boy and I went back an hour and a half later and found that my tray of milk had frozen into ice cream while his was still only a thick liquid not yet frozen.” The gamble paid off, but set in motion a chain of events that went far beyond a cool lick in sweltering Tanga town.

Mpemba had discovered a phenomenon that had been observed before but never explained. Sometimes, against all our intuitions, warm liquid freezes faster than cold.

Aristotle noticed this in the fourth century and wrote: ” … many people when they want to cool water quickly begin by putting it in the sun … “. Mpemba said that the ice cream makers of Tanga were among them. Before ice cream was even thought about in Tanga, and almost two millennia after Aristotle, the French philosopher Descartes observed: “… water which has been kept for a long time on the fire freezes faster than other water”. Twenty years before the English civil war Francis Bacon noted the same. So did scientist Marliani in Renaissance Italy.

But the mystery remained unsolved, tossed aside by the scientific community like a stone in the shoe of the new physics which could not explain it. Mpemba put the stone back.

But before the world would take notice another person entered the story. Six years after Mpemba’s discovery, Denis Osborne, Professor of Physics at the University of Dar es Salaam, went to speak in Mkwawa High School. Osborne recalls in ‘Physics Education’ that he faced a variety of questions from students: “How do I get to University?”, “What does the syllabus have to do with development?” but almost thirty years later one question still sticks in his mind.

“Why did hot water freeze more quickly than cold?” Mpemba was persistent – he had asked this question before. His first physics lesson on arriving at Mkwawa High School was on heat. Mpemba recalls the scene:

“One day as our teacher taught us about Newton’s Law of Cooling, I asked him the question “Please Sir, why is it that when you put both hot milk and cold milk into a refrigerator at the same time, the hot milk freezes first?” The teacher replied: “I do not think so, Mpemba”. “1 continued: It is true, Sir, I have done it myself’. And he said: “The answer I can give is that you were confused.” “I kept on arguing, and the final answer he gave me was that: “Well all I can say is that Mpemba’s physics is not the universal physics”.

“From then onwards if I failed in a problem by making a mistake in looking up the logarithms this teacher used to say: “That is Mpemba’s physics”. And the whole class adopted this, and any time I did something wrong they used to say to me “That is Mpemba’s …. whatever the thing was”.

Despite ridicule Mpemba adopted a pitbull-like persistence. Once he stole into an empty lab and ran through an experiment with beakers of warm and cold water and again found the same result. When Denis Osborne turned up at the school, he thought he might at last get an answer. Mpemba continues: “He first smiled and asked me to repeat the question. After I repeated it he said: “Is it true? Have you done it?” I said: “Yes”. Then he said: “I do not know but I promise to try this experiment when I am back in Dar es Salaam.” Osborne recalls: “It was a surprising question. I thought he was wrong. On my way back to Dar es Salaam in the car I worked out that he must be wrong in theory.” But Osborne held true to his promise Although Mpemba’s classmates would soon be laughing on the other side of their faces, they continued to rib him. “Next day, my classmates in Form 4 were saying to me that I had shamed them by asking that question and that my aim was to ask a question that Dr Osborne would not be able to answer. Some said to me: “But Mpemba, did you understand your chapter on Newton’s law of cooling?” I told them: “Theory differs from practical”. Some said: “We do not wonder, for that was Mpemba’s physics:’

Osborne was surprised when a technician at University College in Dar tried the experiment and he saw the results. He set about trying to explain them, coming up with a few possible explanations, and publishing material in various journals.

Recently the debate was re-ignited in the pages of the ‘New Scientist’ – exposing the same scepticism faced by Mpemba.

Norman Gardiner in Cheshire wondered whether Mpemba knew that his name had entered the language and that scientists were still debating the question that he put to Professor Osborne 30 years later. Charles Knight of the National Centre for Atmospheric Research in Boulder, Colorado recommended readers of the ‘New Scientist’ to read a paper which had been published in ‘The Transactions of the American Philosophical Society’ in 1948 under the heading ‘The Freezing of Supercooled Water’ but J Jocelyn from the Scottish Universities Research and Reactor Centre warned readers that this extended to 80 pages! Tom Trull from the University of Tasmania wrote: “This is a cultural myth.” But his colleague Michael Davis from the same university countered: ” … it is possible to produce ice cubes more quickly by using initially hot water instead of cold”. Matti Jarvilehto at the University of Oulu in Finland. writing in the New Scientist’s ‘Last Word’ section, reported on a similar effect in an electric sauna. “By fooling the temperature sensor by splashing water I increased the oven’s output” he wrote.

With the controversy having been re-opened in Tanzania by Mpemba the solution could come from elsewhere in Africa. South African physicist David Auerbach working in Germany thinks he knows the answer.

“It’s all to do with super cooling” Auerbach was reported as saying in the ‘New Scientist’ in late 1995. After 103 experiments he found that the Mpemba effect is not an iron law – sometimes warm water and sometimes cold water freezes first. The explanation lies in branching ‘ferns’ of ice. These shoot out from the walls of beakers full of water as it just begins to freeze. Auerbach says that this is a sign of super cooling. The Mpemba effect occurs with this super cooling. The sudden appearance of ‘ferns’ of ice crystals from the super cooled liquid occurs at a higher temperature in water that was originally hot. This means that warm water can freeze quicker than cold.

But will Mpemba’s discovery lead to any new inventions? Osborne is not sure. “Among other things the effect is quite erratic. It depends on random vibrations and wafts of air so you get different effects on different occasions.” This could limit its practical applications. Yet whether the Mpemba effect solves the world’s energy crisis or patches the hole in the ozone layer it undoubtedly affected the people involved.

“Surprising questions are fun. They are not to be despised,” says Osborne. “One piece that’s to my credit is that I did not despise, I went off and researched it.” Osborne has pursued a distinguished career in science education. Mpemba’s life has followed a different path since the two parted company in 1969 after they published their joint paper in ‘Physics Education’.

FINDING ERASTO MPEMBA
Finding a one-time physics student in a country the size of Tanzania seemed, at first, like a daunting task. But after tracking down Professor Osborne in Britain, the task became straightforward as they had remained in touch with each other. In Dar es Salaam, after a tip off from a Security guard at the Ministry of Natural Resources and Tourism, we tracked him down to the “Ivory Room” in Changombe, Dar es Salaam. This is where the big annual sale of Tanzanian ivory used to be held before marketing the tusks was banned. Ivory is still stored there from elephants that are damaging crops and have to be killed.

Mpemba’s work with wildlife means that he is no stranger to elephants. “We were on control work in Dodoma some years ago” he said. “We met a big herd. The leader dominating the herd was tuskless. They were ruining peoples crops. He had to go. But he decided to die with us”. “I aimed between the eyes. He went down. He stood up again and charged. I shot him again at 50 yards. I shot him for a third time, a fourth time. A fifth time. He was over my head. As he finally fell, his body grazed my leg.”

But run-ins with elephants were not what Mpemba always had in mind. Originally he wanted to be a doctor but his parents did not have the money to pay for the training, he said. Seeing work with wildlife as the best way to an overseas scholarship he ended up at the Mweka Wildlife College. After getting his diploma he was promoted to Regional Natural Resources Officer in Mara Region in 1967. It was eight years before he achieved his ambition of foreign study. In Australia he got a degree in Natural Resources Management. Today Mpemba is Principal Assistant Game officer working with communities to involve more people in conservation.

Funnily enough his wife is a doctor. But science does not seem to run in the family. “And your children, are there any budding physicists among them?” we asked. “No. They are not doing at all well in physics” he replied. Mpemba may have done more for science than science has done for him. Osborne says: “He is slightly cynical about the whole thing and says people are making too much fuss about it. He did badly in his physics exam”. But if the Mpemba effect does lead to some new advance it will not be a first for Africa. “Some aspects of environmental science and drugs taken from natural sources are being pioneered by Africans and people in other developing countries,” says Osborne.

And Mpemba’s lesson is more than just about the properties of water. How many students in the high tech labs of British schools have made a similar contribution to science understanding? And how many teachers have dismissed students who go against the textbooks? The “black sheep of the class” may have done badly in his physics exam but he has taught a lesson to students and teachers the world over.
Matthew Green

(The heavily guarded Ivory Room is still in use. 144 elephant tusks, disguised as boat spare parts and believed to be from the mainland were recently seized at Zanzibar airport. This brings the total stock of ivory in the Room to some 60 tonnes worth $6 million. The fate of this ivory is to be decided at the Convention on International Trade in Endangered Species meeting in Harare in July – Editor).

1 Comment »

  1. Mpemba’s baffling discovery: can hot water freeze before cold? (1969) | Travel to Kelantan said,

    June 2, 2011 at 11:16 am

    […] Erasto Mpemba, circa 1997 (source). […]

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