...DOES COLD WATER FREEZE FASTER THAN HOT WATER? http://math.ucr.edu/home/baez/physics/General/hot_water.html Hot water seems to freeze faster than cold water, known as the Mpemba effect. The effect was named after the Tanzanian student who in 1963 noticed that hot ice cream mixfreezes faster than a cold one. The effect was first observed by Aristotle in the 4th century BC, then later Francis Bacon and René Descartes. This seems impossible, right? Many of you may have already come up with a common proof that the Mpemba effect is impossible. The proof usually goes something like this. Say that the initially cooler water starts at 30°C and takes 10 minutes to freeze, while the initially warmer water starts out at 70°C. Now the initially warmer water has to spend some time cooling to get to get down to 30°C, and after that, it's going to take 10 more minutes to freeze. So since the initially warmer water has to do everything that the initially cooler water has to do, plus a little more, it will take at least a little longer, right? What can be wrong with this proof? What's wrong with this proof is that it implicitly assumes that the water is characterized solely by a single number — its average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30°C, it may look very different than the initially cooler water (at a uniform 30°C) did at the start. Why? Because the water...
Words: 984 - Pages: 4
...aside, Mpemba was mocked by his teacher for claiming hot water freezes faster than cold water because it seemed to contradict thermodynamics. Moral of the story? Don't be so fast to dismiss the observations of a non-scientist just because they don't agree with what you think you know. In fact, that lesson probably applies to most academic disciplines and, perhaps, all of human knowledge. Skepticism of people's claims is important. The Mpemba effect occurs when two bodies of water, identical in every way, except that one is at a higher temperature than the other, are exposed to identical subzero surroundings, and the initially hotter water freezes first. The effect appears theoretically impossible at first sight. Nevertheless, it has been observed in numerous experiments, and we will see that it is in fact possible. Readers who are quite certain that the effect is forbidden by the laws of thermodynamics should pause for a moment to do two things. First, to try to explain, as precisely as possible, why it is impossible. And second, to decide how to respond to a non-scientist who insists that they have observed the Mpemba effect in a home experiment. Whether or not the effect is real, careful consideration of these tasks will bring up a wealth of important issues about the scientific method, that can be understood and discussed by students without any knowledge of advanced physics... To see how the effect might occur, it is useful to carefully think about why the effect appears...
Words: 571 - Pages: 3
...The Mpemba effect describes the phenomenon that hot water may freeze faster than cold. To precisely state what is Mpemba impact, begin with two containers of water, same in shape, and with same amounts of water. The only difference is the water temperature one is at a higher temperature than the water in the other. cool two containers, using the exact same cooling process for both containers. In some conditions at first warmer water will freeze first. It means the Mpemba effect happens. Here's the proof. Say that it takes 10 minutes to freeze the initially cooler water starts at 30°C , while the initially warmer water starts out at 70°C. Now the initially warmer water needs more cooling time to get to get down to 30°C, then it needs 10 more minutes to freeze. So since the initially warmer water has to do everything that the initially cooler water has to do, plus a little more, it will take at least a little longer. Somehow, the hot water must be able to lose its heat faster than the cold water. Evaporation- Theoretical calculations have shown that evaporation can explain the Mpemba effect if you assume that the water loses heat solely through evaporation . When the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less...
Words: 530 - Pages: 3
