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Decker, a meteorologist at Oregon State University in Corvallis, encourages readers to settle the question for themselves: "You can readily set up an experiment to learn which freezes earlier: water that is initially hot, or water that is initially cold. Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? His question to guest lecturer Dr. Denis G. It's completely counterintuitive and seems to violate the basic laws of thermodynamics.
To be clear, what we're saying here is that under certain conditions, the total time it takes for a volume of warm water to freeze will be smaller than the total time it takes for an equal volume of cold water to freeze, given the exact same external temperature.
It's a really strange thing. I mean, at some point in the process, doesn't the warm water reach the exact same initial state as the cold water? And if so, why does that cold-water-that-was-recently-hot freeze faster than the water that started out cold? It's left folks scratching their heads or outright denying its existence for decades.
Since then, numerous explanations have been put forth to try and explain the phenomenon, but none have been much more than plausible-sounding theories. Here are a few of them:.
Theory: Convection currents in the warm water caused by large temperature differentials will cause it to cool more rapidly, and those convection currents continue even after the water has dropped to the same temperature as the cooler water, thus allowing it to overtake the cooler water in freezing. Theory: Hot water evaporates.
Less water left behind means less water to freeze. The teacher answered that Mpemba must have been confused. When Mpemba kept arguing, the teacher said "All I can say is that is Mpemba's physics and not the universal physics" and from then on, the teacher and the class would criticize Mpemba's mistakes in mathematics and physics by saying "That is Mpemba's mathematics" or "That is Mpemba's physics.
Earlier, Dr Osborne, a professor of physics, had visited Mpemba's high school. Mpemba had asked him to explain why hot water would freeze before cold water. Dr Osborne said that he could not think of any explanation, but would try the experiment later. When back in his laboratory, he asked a young technician to test Mpemba's claim. The technician later reported that the hot water froze first, and said "But we'll keep on repeating the experiment until we get the right result.
In the same year, in one of the coincidences so common in science, Dr Kell independently wrote a paper on hot water freezing sooner than cold water.
Kell showed that if one assumed that the water cooled primarily by evaporation, and maintained a uniform temperature, the hot water would lose enough mass to freeze first [11]. Kell thus argued that the phenomenon then a common urban legend in Canada was real and could be explained by evaporation. But he was unaware of Osborne's experiments, which had measured the mass lost to evaporation and found it insufficient to explain the effect.
Subsequent experiments were done with water in a closed container, eliminating the effects of evaporation, and still found that the hot water froze first [14]. Subsequent discussion of the effect has been inconclusive.
While quite a few experiments have replicated the effect [4,6—13] , there has been no consensus on what causes the effect. The different possible explanations are discussed above. The effect has repeatedly a topic of heated discussion in the "New Scientist", a popular science magazine.
The letters have revealed that the effect was known by laypeople around the world long before Today, there is still no well-agreed explanation of the Mpemba effect. One explanation of the effect is that as the hot water cools, it loses mass to evaporation. With less mass, the liquid has to lose less heat to cool, and so it cools faster.
With this explanation, the hot water freezes first, but only because there's less of it to freeze. Calculations done by Kell in [11] showed that if the water cooled solely by evaporation, and maintained a uniform temperature, the warmer water would freeze before the cooler water.
This explanation is solid, intuitive, and undoubtedly contributes to the Mpemba effect in most physical situations. But many people have incorrectly assumed that it is therefore "the" explanation for the Mpemba effect. That is, they assume that the only reason hot water can freeze faster than cold is because of evaporation, and that all experimental results can be explained by the calculations in Kell's article. But the experiments currently do not bear this belief out.
While experiments show evaporation to be important [13] , they do not show that it is the only mechanism behind the Mpemba effect. A number of experimenters have argued that evaporation alone is insufficient to explain their results [5,9,12] ; in particular, the original experiment by Mpemba and Osborne measured the mass lost to evaporation, and found it substantially less that the amount predicted by Kell's calculations [5,9].
And most convincingly, an experiment by Wojciechowski observed the Mpemba effect in a closed container, where no mass was lost to evaporation. Another explanation argues that the dissolved gas usually present in water is expelled from the initially hot water, and that this changes the properties of the water in some way that explains the effect.
It has been argued that the lack of dissolved gas may change the ability of the water to conduct heat, or change the amount of heat needed to freeze a unit mass of water, or change the freezing point of the water by some significant amount.
It is certainly true that hot water holds less dissolved gas than cold water, and that boiled water expels most dissolved gas.
The question is whether this can significantly affect the properties of water in a way that explains the Mpemba effect. As far as I know, there is no theoretical work supporting this explanation for the Mpemba effect. Indirect support can be found in two experiments that saw the Mpemba effect in normal water which held dissolved gasses, but failed to see it when using degassed water [10,14].
But an attempt to measure the dependence of the enthalpy of freezing on the initial temperature and gas content of the water was inconclusive [14]. One problem with this explanation is that many experiments pre-boiled both the initially hot and initially cold water, precisely to eliminate the effect of dissolved gasses, and yet they still saw the effect [5,13].
Two somewhat unsystematic experiments found that varying the gas content of the water made no substantial difference to the Mpemba effect [9,12]. It has also been proposed that the Mpemba effect can be explained by the fact that the temperature of the water becomes non-uniform.
As the water cools, temperature gradients and convection currents will develop. For most temperatures, the density of water decreases as the temperature increases. So over time, as water cools we will develop a "hot top" — the surface of the water will be warmer than the average temperature of the water, or the water at the bottom of the container. If the water loses heat primarily through the surface, then this means that the water should lose heat faster than one would expect based just on looking at the average temperature of the water.
And for a given average temperature, the heat loss should be greater the more inhomogenous the temperature distribution is that is, the greater the range of the temperatures seen as we go from the top to the bottom. How does this explain the Mpemba effect? But logic triumphs when it comes the plain ordinary water that comes from the household faucet. Most likely to impact the freezing point of water is the presence of impurities such as salt, dissolved solids and gases — and the ingredients of homemade ice cream.
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