Problem so that any laboratory undertaking such investigations is guaranteed different As Firth wrote "There is a wealth of experimental variation in the Because of this sensitivity, whileĮxperiments have generally agreed that the Mpemba effect occurs, they disagree over theĬonditions under which it occurs, and thus about why it occurs.
The time of freezing is defined, and so on. Shape and size of the refrigeration unit, the gas and impurity content of the water, how Of details in the experimental setup, such as the shape and size of the container, the The main problem is that the time it takes water to freeze is highly sensitive to a number Why hasn't modern science answered this seemingly simple question about cooling water? Selective in which experimental results you trust the problem is that different people Of course, there is nothing wrong with informed theoretical guesswork or being Hear confident claims that X is the cause of the Mpemba effect, such claims are usuallyīased on guesswork, or on looking at the evidence in only a few papers and ignoring the If any, of the proposed mechanisms is the most important one. A number of possible explanationsįor the effect have been proposed, but so far the experiments do not show clearly which, It is still not known exactly why this happens. Observed in a number of controlled experiments Impossibility proof given above doesn't work. All four of these changesĪre conceivably important, and each will be considered separately below. The environment around the container in the refrigerator. It could have less mass, less dissolved gas, or convectionĬurrents producing a non-uniform temperature distribution. Why?īecause the water may have changed when it cooled down from a uniform 70☌ to anĪverage 30☌. Than the initially cooler water (at a uniform 30☌) did at the start. Warmer water has cooled to an average temperature of 30☌, it may look very different Other factors besides the average temperature are important, then when the initially What's wrong with this proof is that it implicitly assumes that the water isĬharacterized solely by a single number - its average temperature. More, it will take at least a little longer, right? What can be wrong with this
#Throwing hot water into cold air video plus
Warmer water has to do everything that the initially cooler water has to do, plus a little Initially warmer water has to spend some time cooling to get to get down to 30☌, andĪfter that, it's going to take 10 more minutes to freeze. Minutes to freeze, while the initially warmer water starts out at 70☌. Say that the initially cooler water starts at 30☌ and takes 10 This seems impossible, right? Many sharp readers may have already come up with aĬommon proof that the Mpemba effect is impossible. Not see the Mpemba effect for just any initial temperatures, container shapes, or cooling Will freeze first: if that happens, you have seen the Mpemba effect. But under some conditions the initially warmer water The cold water at 0.01☌, then clearly under those circumstances, the initially cooler Of course, the initially warmer water will not freeze before the initiallyĬooler water for all initial conditions. If this occurs, we have seen the MpembaĮffect. Initially warmer water will freeze first. Using the exact same cooling process for each container. (uniform) temperature than the water in the other. The only difference between the two is that the water in one is at a higher Two containers of water, which are identical in shape, and which hold identical amounts of
Should begin by stating precisely what we mean by the Mpemba effect. Because, no doubt, most readers are extremely skeptical at this point, we The phenomenon that hot water may freeze faster than cold is often called the MpembaĮffect. Parable against making snap judgements about what is impossible. Interesting in their own right - Mpemba's story in particular providing a dramatic Scientific history of this effect, and the story of Mpemba's rediscovery of it, are While this phenomenon has been knownįor centuries, and was described by Aristotle, Bacon, and Descartes, it was not introduced to the modern scientificĬommunity until 1969, by a Tanzanian high school pupil named Mpemba. It has been seen and studied in numerous experiments. This phenomenon is extremely counterintuitive, and surprising even to Hot water can in fact freeze faster than cold water for a wide range of experimentalĬonditions. Can hot water freeze faster than cold water?ĭepartment of Physics, University of California, 1998.Ĭan hot water freeze faster than cold water? Yes - a general explanation History of the Mpemba Effect More-detailed explanations References Yes - a general explanation
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