Aristotle wrote:
"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, for they use the ice like lead to fix the reeds"
Meteorology 1, part 12
Surprisingly, this wasn't observed or documented in modern scientific works until a thirteen-year-old Tanzanian middle-school student named Erasto Barthlomeo Mpemba made an observation in cookery classes that hot ice-cream mixes froze faster than cold ones. After reporting this to his physics teacher and then to a visiting professor from Dar Es Salaam, the observation was confirmed and published in 1969.
Besides for this appearing to be counter-intuitive and contrary to thermodynamics, it is even more surprising that there is still no single conclusive scientific explanation (although several physical theory effects do contribute to the phenomenon including conduction, evaporation, convection and dissolved gases).
At first glance, I would think this might be a function of less molecular density in the warmer substance that would contribute more surface area for the cold air to freeze the water and although it cools gradually and also reaches the temperature of the colder water at some point, which would mean a slower cooling rate and should give the cool water a head-start advantage over the warmer water, somehow (this is where I get stuck), the initial freezing effect continues at the faster rate similar to the rate it started cooling at and becomes frozen before the cooler water that starts out at a slower freezing rate.
Anyway, several people, looking for a moment of internet fame, decided to throw some hot water or coffee into the air to demonstrate the effect. They're all over youtube, here's one example:
No comments:
Post a Comment