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What I've Learned
In 1963 at the Magamba Secondary School in Tanzania, boys would often make their own ice cream by boiling milk, adding sugar, letting it cool to room temperature, and putting it in the freezer compartment of the school's refrigerator.
Space in the freezer was at a premium and was on a first come basis.
One day, 13-year-old Erasto Mpemba [em-PEM-bah] bought some milk and began to boil it. There were only two spaces left in the freezer and another boy, when he saw Mpemba boiling milk, was afraid of loosing a spot so he put his milk and sugar mixture in without boiling it.
Mpemba, fearing someone else would take the last spot before his milk cooled to room temperature, put his mixture in hot. This was believed to be bad for the refrigerator as it might over-work the freezing unit and ruin it, but he did it anyway.
An hour and a half later, both boys showed up at the fridge. Mpemba's hot mixture was frozen into ice cream. The other boy's, which had been put in at room temperature, was a thick liquid, not yet frozen.
Mpemba asked his physics teacher why it happened like that, the hot milk freezing first.
The teacher said, “You were confused, that cannot happen."
Later, when Mpemba was at Mkwawa High School and his science class was studying Newton’s law of cooling, he asked his teacher why hot milk would freeze faster than cool milk.
The teacher told him that such a thing could not happen.
“It is true, sir," the boy said. "I have done it myself.”
The teacher replied that perhaps such a thing was possible in "Mpemba physics," but not in real physics.
The phrase caught on with his classmates. Whenever he made a mistake in any class, they would laugh and call it Mpemba mathematics or Mpemba grammar or Mpemba history.
One day, when no one else was in the school lab, Mpemba took two equal-sized containers--one filled with cool water from the tap and one with boiling water--and put them in a freezer. When he checked back an hour later, neither water had frozen, but there seemed to be more ice crystal forming in the hot container than in the cool one.
Mpemba decided to try the experiment again when he had a chance.
Before he got the chance, however, Dr. Denis G. Osborne, a college physics professor, came to lecture at Mkwawa High School. The students were allowed to ask questions, and you can guess what Mpemba wanted to know.
"If you take two similar containers with equal volumes of water, one at 35 degrees Celsius and the other at 100 degrees Celsius, and put them into a refrigerator, the one that started at 100 degrees Celsius freezes first. Why?"
Professor Osborne smiled and asked Mpemba to repeat the question.
The professor, instead of saying that's impossible, asked, "Is it true, have you done it?"
Osborne said he didn't know why such a thing would happen, but that he would try the experiment himself when back at his college.
Mpemba, it was felt, had shamed his class. He had deliberately asked a stupid question of an eminent scientist, making them all look bad.
Professor Osborne, however, was true to his word. Here's what happened:
"At the University College in Dar es Salaam I asked a young technician to test the facts. The technician reported that the water that started hot did indeed freeze first and added in a moment of unscientific enthusiasm: 'But we’ll keep on repeating the experiment until we get the right result.'"
(The idea of having a "right result" in mind and repeating experiments – tweaking them if necessary – until they conform to that preconceived "right result" is a plague on science today. This type of hocus pocus is easy to pull off if your experiments consist of running computer models. But I digress.)
Eventually, Osborne and Mpemba co-authored a paper about this phenomenon. The fact that, under certain circumstances, hot liquids will freeze faster than cool ones has come to be known as the Mpemba Effect. Even now, the exact reasons for it are not understood. Some scientists attribute it to convective heat transfer, some to evaporation, some to degassing, some to distribution of solutes. These are but a few of the theories.
Mpemba did not become a physicist. Instead, he studied wildlife management and became the principal game officer for the Tanzanian Ministry of Natural Resources and Tourism.
Erasto Mpemba is a hero of mine. So is Professor Denis G. Osborne.