Four tubes filled with water. Three of them have water above 60°C [140°F], but in one of them it’s only at 28,5°C [83°F].
Answer quickly: which freezes first? Don’t worry, this is not a silly tricky question.
It’s a tricky, baffling physical phenomenon, because the tube where water was initially at 60,5 °C freezes first, followed by the ones at just above 63°C, and a long time later, the one that was at a comfortable 28,5°C. Warm water freezes faster than cold water, on what’s known as the Mpemba Effect .
This is truly a very nice effect to know, because first of all, it’s still not completely understood. Many explanations have been proposed, and they all probably play a role on it. The controversy goes on, and the Straight Dope even plays it down. Poor Mpemba. The story of the discovery is adorable. From Wikipedia:
“High-school student Erasto B. Mpemba first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School, Tanzania when freezing hot ice cream mix in cookery classes and noticing that they froze before cold mixes. After passing his O-level examinations, he became a student at Mkwawa Secondary (formerly High) School, Iringa, Tanzania. The headmaster invited Dr. Denis G. Osborne from the University College in Dar Es Salaam to give a lecture on Physics. After the lecture, Erasto Mpemba asked him the question “If you take two similar containers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a refrigerator, the one that started at 100°C freezes first. Why?” only to be ridiculed by his classmates. After initial consternation, Dr. Osborne confirmed Erasto’s finding and they published the results together in 1969.”
Tamás Jávor also writes a longer and more delicious version of the story here (scroll down). A story as adorable, perhaps even more adorable than the often referenced story of Emily Rosa. Rosa is the then 9-year-old girl from Colorado who published what began as her school project on the Journal of the American Medical Association, demonstrating how therapeutic touch didn’t pass even the most basic test. An inspiring and nice story for the young critical thinkers everywhere.
But Mpemba is from Tanzania! And he seemingly questioned the fundamentals of physics with ice cream! I vote for Mpemba.
More than adorable, enlightening and intriguing, the Mpemba effect should also be better known because it’s in fact known for more than two thousand years. You read it right: Erasto Mpemba rediscovered an effect that had been described by Aristotle, circa 350 BCE:
“The fact that the 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. So the inhabitants of Pontus when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their reeds that it may freeze the quicker”
Not only Aristotle. Amazingly, it was common knowledge to Francis Bacon (“slightly tepid water freezes more easily than that which is utterly cold“) and even René Descartes (“One can see by experience that water that has been kept on a fire for a long time freezes faster than other“)!
How come? Charles Fort would delight on this case. It just proves his basic point. Following the Scientific Revolution, with Newton, and the gradual formulation of the main pillar of physical science, thermodynamics, the then Aristotle-Bacon-Descartes Effect didn’t make any sense. You have to spend more energy to cool hot water. That’s a proven fact, and this knowledge applies successfully to almost everything we use today, such as our cars. Ergo, Descartes be damned, hot water takes more time to freeze than water.
But let’s not burn science books yet. The fact the effect is not linear, the room temperature, and the availability of freezing equipment may also have played a big part here. If you follow the graphic above, you may see that up until around 25 degrees, cold water did freeze faster than warmer water, and even going a bit beyond that, dealing with room temperature, the effect is still not that clear — hot water may take as much time to freeze as slightly colder water. Refrigerators were also not in widespread use until the twentieth century. Finally, to get a nice graphic like the one above, you have to control many variables carefully. Which explains why Cecil Adams, of the Straight Dope, didn’t see it in his home freezer. Just check this other, more recent graphic, involving different test conditions (click for explanation). Not that simple.
Thermodynamics, by the way, is still valid, as far as we know. You still have to spend more energy to cool hot water. The Mpemba Effect, almost certainly, has something to do with freezing and heat conduction, not just cooling, and the many physical anomalies of water. Besides, Aristotle, Bacon and Descartes were not that right either: they mentioned the effect, but explained it away in plainly wrong terms.
There are lessons for everyone here, but the main one is certainly that Nature is still master, reserving many surprises, even if they are more than two thousand years old.
Oh, but one last Fortean bit. Remember how Mpemba fortuitously rediscovered the effect with ice cream, having the support of Dr. Osborne, both from Tanzania, leading to publication on the Physics Education journal in May 1969? Interestingly, another modern scientist independently rediscovered and published about the phenomenon. On the same year. On the same month. In the May 1969 American Journal of Physics, Canadian scientist George Kell also published a paper on “the freezing of hot and cold water“.
Mpemba and Osborne started their work in 1963, while Kell reported his paper in November 1968, before any publication. They were, unknowingly to each other, rehabilitating this damned data and got finally published at almost the exact same time. How cool is that?
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