Why does hot water freeze faster than cold?

Lifelong freezer riddle



Why does hot water freeze faster than cold?
THE letters have arrived during most of Erasto Mpemba’s life. Some have been from physicists, who found his address through Tanzanian embassies.
Some were from students, wanting to know if he had found a solution to his conundrum yet.
Sometimes, during Mr Mpemba’s career in game park management, he would disappear for extended periods — when he was free from having to answer these missives on why hot water freezes faster than cold.
But the letters kept coming. “When I came back from the bush,” he says of one occasion, “there were 40 letters.”
Last week, however, was unprecedented. Because, after The Times publicised a call by the Royal Society of Chemistry for solutions to the Mpemba effect — the counterintuitive freezing phenomenon he discovered 50 years ago when still a schoolboy — there have been 22,000 letters and e-mails.

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So it was that, six months later, Mr Mpemba was preparing to be guest of honour at the announcement of the winner of the competition. With him were a physics professor and a former technician, the only people who took his theories seriously. And a knickerbocker glory.
Which was appropriate really, because in the early 1960s Mr Mpemba noticed that boiling milk froze faster in a freezer than milk put in at room temperature.
He asked his physics teacher why this would be the case. The answer was simple. “He told me it wasn’t,” he said, picking a strawberry off the top of his ice cream.
A bit later he moved to a different school and teacher, but the reaction was the same. “He would say, again and again, ‘You still don’t know Newton’s law of cooling’.”
When Mr Mpemba insisted, “the teacher said, ‘There is Mpemba’s physics and universal physics’.” This became a refrain whenever he got anything wrong. “It pained me.”
But then he heard that Denis Osborne, a physics professor from Dar es Salaam, now living in Canterbury, was visiting. “I thought, ‘Ah, this is my chance. This man knows more than my teacher’.”
However, his classmates were equally determined that his obsession would not embarrass them. Ray deSouza was a laboratory technician travelling with Professor Osborne. He remembers well the reaction to the question.
“There was hollering, they clearly didn’t want him to ask it. He was very brave. Denis listened though. He said, ‘Have you done the experiment?’ Mpemba replied in a very strong voice, ‘Yes, yes’.” But even as Mr Mpemba was getting told off, Professor Osborne was planning to repeat his experiment.
“I thought, it’s impossible, it’s wrong,” Professor Osborne said. But, nevertheless, he told Mr deSouza to try it.
Some days later, “one of the other technicians met me in the corridor and said, ‘Professor, we keep trying the experiment and we keep getting the wrong result’.”
Whenever they tried it, the hot water froze first — but it wasn’t the wrong result at all. In 1969 Mr Mpemba and Professor Osborne published a paper on “the Mpemba effect”. Scientists, and 22,000 entrants to the RSC’s competition, have been trying ever since to explain it.
Why has it captured people’s imagination? “It’s everyday science; we all have freezers, and it’s a very surprising effect,” Professor Osborne said. “It is also a good story for students. It encourages them to think outside the box and defy authority. Mr Mpemba,” he said, looking at his protégé, now 67 and with a slightly greying beard, “was a very stubborn boy.”
The RSC’s winning entry was from Nikola Bregovic, a chemist from Croatia, who posits that the effect is because of convection currents and a phenomenon known as supercooling. But the chances are that is still not a definitive answer.
As far as Mr Mpemba is concerned, that’s fine. He returns to Tanzania this week and says of the continued mystery: “It’s kept me busy writing to people.”
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