Can hot water freeze faster than cold water? Written Nov, 1998 by Monwhea Jeng (Momo), Department of Physics, University of California Can hot water freeze faster than cold water? Yes — a general explanation Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments . While this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1–3] , it was not introduced to the modern scientific community until 1969, by a Tanzanian high school pupil named Mpemba. Both the early scientific history of this effect, and the story of Mpemba's rediscovery of it, are interesting in their own right — Mpemba's story in particular providing a dramatic parable against making snap judgements about what is impossible. This is described separately below. The phenomenon that hot water may freeze faster than cold is often called the Mpemba effect.

Because, no doubt, most readers are extremely skeptical at this point, we should begin by stating precisely what we mean by the Mpemba effect. We start with two containers of water, which are identical in shape, and which hold identical amounts of water. The only difference between the two is that the water in one is at a higher (uniform) temperature than the water in the other. Now we cool both containers, using the exact same cooling process for each container. Under some conditions the initially warmer water will freeze first. If this occurs, we have seen the Mpemba effect. Of course, the initially warmer water will not freeze before the initially cooler water for all initial conditions. If the hot water starts at 99.9°C, and the cold water at 0.01°C, then clearly under those circumstances, the initially cooler water will freeze first. However, under some conditions the initially warmer water will freeze first: if that happens, you have seen the Mpemba effect. But you will not see the Mpemba effect for just any initial temperatures, container shapes, or cooling conditions. This seems impossible, right? Many sharp readers 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 may have changed when it cooled down from a uniform 70°C to an average 30°C. It could have less mass, less dissolved gas, or convection currents producing a non-uniform temperature distribution.

Or it could have changed the environment around the container in the refrigerator. All four of these changes are conceivably important, and each will be considered separately below. So the impossibility proof given above doesn't work. And in fact the Mpemba effect has been observed in a number of controlled experiments [5,7–14] It is still not known exactly why this happens. A number of possible explanations for the effect have been proposed, but so far the experiments do not show clearly which, if any, of the proposed mechanisms is the most important one. While you will often hear confident claims that X is the cause of the Mpemba effect, such claims are usually based on guesswork, or on looking at the evidence in only a few papers and ignoring the rest. Of course, there is nothing wrong with informed theoretical guesswork or being selective in which experimental results you trust; the problem is that different people make different claims as to what X is. Why hasn't modern science answered this seemingly simple question about cooling water?

The main problem is that the time it takes water to freeze is highly sensitive to a number of details in the experimental setup, such as the shape and size of the container, the shape and size of the refrigeration unit, the gas and impurity content of the water, how the time of freezing is defined, and so on. Because of this sensitivity, while experiments have generally agreed that the Mpemba effect occurs, they disagree over the conditions under which it occurs, and thus about why it occurs. As Firth [7] wrote "There is a wealth of experimental variation in the problem so that any laboratory undertaking such investigations is guaranteed different results from all others." So with the limited number of experiments done, often under very different conditions, none of the proposed mechanisms can be confidently proclaimed as "the" mechanism. Above we described four ways in which the initially warmer water could have changed upon cooling to the initial temperature of the initially cooler water. What follows below is a short description of the four related mechanisms that have been suggested to explain the Mpemba effect. More ambitious readers can follow the links to more complete explanations of the mechanisms, as well as counter-arguments and experiments that the mechanisms cannot explain.

It seems likely that there is no one mechanism that explains the Mpemba effect for all circumstances, but that different mechanisms are important under different conditions. Evaporation — As 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 ice. Theoretical calculations have shown that evaporation can explain the Mpemba effect if you assume that the water loses heat solely through evaporation [11] . This explanation is solid, intuitive, and evaporation is undoubtedly important in most situations. However, it is not the only mechanism. Evaporation cannot explain experiments that were done in closed containers, where no mass was lost to evaporation [12] . And many scientists have claimed that evaporation alone is insufficient to explain their results [5,9,12] . Dissolved Gasses — Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water.

It has been speculated that this changes the properties of the water in some way, perhaps making it easier to develop convection currents (and thus making it easier to cool), or decreasing the amount of heat required to freeze a unit mass of water, or changing the boiling point. There are some experiments that favor this explanation [10,14] , but no supporting theoretical calculations. Convection — As the water cools it will eventually develop convection currents and a non-uniform temperature distribution. At most temperatures, density decreases with increasing temperature, and so the surface of the water will be warmer than the bottom: this has been called a "hot top." Now if the water loses heat primarily through the surface, then water with a "hot top" will lose heat faster than we would expect based on its average temperature. When the initially warmer water has cooled to an average temperature the same as the initial temperature of the initially cooler water, it will have a "hot top", and thus its rate of cooling will be faster than the rate of cooling of the initially cooler water at the same average temperature. Got all that? You might want to read this paragraph again, paying careful distinction to the difference between initial temperature, average temperature, and temperature.

While experiments have seen the "hot top", and related convection currents, it is unknown whether convection can by itself explain the Mpemba effect. Surroundings — A final difference between the cooling of the two containers relates not to the water itself, but to the surrounding environment. The initially warmer water may change the environment around it in some complex fashion, and thus affect the cooling process. For example, if the container is sitting on a layer of frost which conducts heat poorly, the hot water may melt that layer of frost, and thus establish a better cooling system in the long run. Obviously explanations like this are not very general, since most experiments are not done with containers sitting on layers of frost. Finally, supercooling may be important to the effect. Supercooling occurs when the water freezes not at 0°C, but at some lower temperature. One experiment [12] found that its initially hot water supercooled less than its initially cold water. This would mean that the initially warmer water might freeze first because it would freeze at a higher temperature than the initially cooler water. If true, this would not fully explain the Mpemba effect, because we would still need to explain why initially warmer water supercools less than initially cooler water.

In short, hot water does freeze sooner than cold water under a wide range of circumstances. It is not impossible, and has been seen to occur in a number of experiments. However, despite claims often made by one source or another, there is no well-agreed explanation for how this phenomenon occurs. Different mechanisms have been proposed, but the experimental evidence is inconclusive. For those wishing to read more on the subject, Jearl Walker's article in Scientific American [13] is very readable and has suggestions on how to do home experiments on the Mpemba effect, while the articles by Auerbach [12] and Wojciechowski [14] are two of the more modern papers on the effect. History of the Mpemba Effect The fact that hot water freezes faster than cold has been known for many centuries. The earliest reference to this phenomenon dates back to Aristotle in 300 B.C. The phenomenon was later discussed in the medieval era, as European physicists struggled to come up with a theory of heat. But by the 20th century the phenomenon was only known as common folklore, until it was reintroduced to the scientific community in 1969 by Mpemba, a Tanzanian high school pupil.

Since then, numerous experiments have confirmed the existence of the "Mpemba effect", but have not settled on any single explanation. The earliest known reference to this phenomenon is by Aristotle, who wrote: "The fact that 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. . ." [1,4] He wrote these words in support of a mistaken idea which he called antiperistasis. Antiperistasis is defined as "the supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality, for instance, the sudden heating of a warm body when surrounded by cold" [4] . Medieval scientists believed in Aristotle's theory of antiperistasis, and also sought to explain it. Not surprisingly, scientists in the 1400s had trouble explaining how it worked, and could not even decide whether (as Aristotle claimed in support of antiperistasis), human bodies and bodies of water were hotter in the winter than in the summer [4] .

Around 1461, the physicist Giovanni Marliani, in a debate over how objects cooled, said that he had confirmed that hot water froze faster than cold. He said that he had taken four ounces of boiling water, and four ounces of non-heated water, placed them outside in similar containers on a cold winter day, and observed that the boiled water froze first. Marliani was, however, unable to explain this occurrence [4] . Later, in the 1600s, it was apparently common knowledge that hot water would freeze faster than cold. In 1620 Bacon wrote "Water slightly warm is more easily frozen than quite cold" [2] , while a little later Descartes claimed "Experience shows that water that has been kept for a long time on the fire freezes sooner than other water" [3] . In time, a modern theory of heat was developed, and the earlier observations of Aristotle, Marliani, and others were forgotten, perhaps because they seemed so contradictory to modern concepts of heat.

However, it was still known as folklore among many non-scientists in Canada [11] , England [15–21] , the food processing industry [23] , and elsewhere. It was not reintroduced to the scientific community until 1969, 500 years after Marliani's experiment, and more than two millennia after Aristotle's "Meteorologica I" [1] . The story of its rediscovery by a Tanzanian high school pupil named Mpemba is written up in the New Scientist [4] . The story provides a dramatic parable cautioning scientists and teachers against dismissing the observations of non-scientists and against making quick judgements about what is impossible. In 1963, Mpemba was making ice cream at school, which he did by mixing boiling milk with sugar. He was supposed to wait for the milk to cool before placing it the refrigerator, but in a rush to get scarce refrigerator space, put his milk in without cooling it. To his surprise, he found that his hot milk froze into ice cream before that of other pupils. He asked his physics teacher for an explanation, but was told that he must have been confused, since his observation was impossible.

Mpemba believed his teacher at the time. But later that year he met a friend of his who made and sold ice cream in Tanga town. His friend told Mpemba that when making ice cream, he put the hot liquids in the refrigerator to make them freeze faster. Mpemba found that other ice cream sellers in Tanga had the same practice. Later, when in high school, Mpemba learned Newton's law of cooling, that describes how hot bodies are supposed to cool (under certain simplifying assumptions). Mpemba asked his teacher why hot milk froze before cold milk when he put them in the freezer. 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." But when Mpemba later tried the experiment with hot and cold water in the biology laboratory of his school, he again found that the hot water froze sooner.

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." However, repeated tests gave the same result, and in 1969 Mpemba and Osborne wrote up their results [5] . 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. However, 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 1969. Today, there is still no well-agreed explanation of the Mpemba effect. Evaporation 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 1969 [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. However, 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. However, the experiments currently do not bear out this belief. 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.

Dissolved Gasses 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] . However, 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] . Convection 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? Well, the initially hot water will cool rapidly, and quickly develop convection currents and so the temperature of the water will vary greatly from the top of the water to the bottom. On the other hand, the initially cool water will have a slower rate of cooling, and will thus be slower to develop significant convection currents. Thus, if we compare the initially hot water and initially cold water at the same average temperature, it seems reasonable to believe that the initially hot water will have greater convection currents, and thus have a faster rate of cooling. To consider a concrete example, suppose that the initially hot water starts at 70°C, and the initially cold water starts at 30°C. When the initially cold water is at an average 30°C, it is also a uniform 30°C.

However, when the initially hot water reaches an average 30°C, the surface of the water is probably much warmer than 30°C, and it will thus lose heat faster than the initially cold water for the same average temperature. Got that? This explanation is pretty confusing, so you might want to go back and read the last two paragraphs again, paying careful attention to the difference between initial temperature, average temperature, and surface temperature. At any rate, if the above argument is right, then when we plot the average temperature versus time for both the initially hot and initially cold water, then for some average temperatures the initially hot water will be cooling faster than the initially cold water. So the cooling curve of the initially hot water will not simply reproduce the cooling curve of the initially cold water, but will drop faster when in the same temperature range. This shows that the initially hot water goes faster, but of course it also has farther to go. So whether it actually finishes first (that is, reaches 0°C first), is not clear from the above discussion. To know which one finishes first would require theoretical modelling of the convection currents (hopefully for a range of container shapes and sizes), which has not been done.

So convection alone may be able to explain the Mpemba effect, but whether it actually does is not currently known. Experiments on the Mpemba effect have often reported a "hot top" [5,8,10] , as we would expect. Experiments have been done that looked at the convection currents of freezing water [27,28] , but their implications for the Mpemba effect are not entirely clear. It should also be noted that the density of water reaches a maximum at four° C. So below four°C, the density of water actually decreases with decreasing temperature, and we will get a "cold top." This makes the situation even more complicated. Surroundings The initially hot water may change the environment around it in some way that makes it cool faster later on. One experiment reported significant changes in the data simply upon changing the size of the freezer that the container sat in [7] . So conceivably it is important not just to know about the water and the container, but about the environment around it.

For example, one explanation for the Mpemba effect is that if the container is resting on a thin layer of frost, than the container holding the cold water will simply sit on the surface of the frost, while the container with the hot water will melt the frost, and then be sitting on the bottom of the freezer. The hot water will then have better thermal contact with the cooling systems. If the melted frost refreezes into an ice bridge between the freezer and the container, the thermal contact may be even better. Obviously, even if this argument is true, it has fairly limited utility, since most scientific experiments are careful enough not to rest the container on a layer of frost in a freezer, but instead place the container on a thermal insulator, or in a cooling bath. So while this proposed mechanism may or may not have some relevance to some home experiments, it's irrelevant for most published results. Supercooling Finally, supercooling may be important to the effect. Supercooling occurs when water freezes not at 0°C, but at some lower temperature.

This happens because the statement that "water freezes at 0°C" is a statement about the lowest energy state of the water: at less than 0°C, the water molecules "want" to be arranged as an ice crystal. This means that they will stop zooming around randomly as a liquid, and instead form a solid ice lattice. However, they don't know how to form themselves into an ice lattice, but need some small irregularity or nucleation site to tell them how to arrange themselves. Sometimes, when water is cooled below 0°C, the molecules will not see a nucleation site for some time, and then water will cool below 0°C without freezing. This happens quite often. One experiment found that initially hot water would supercool only a little (say to about −2°C), while initially cold water would supercool more (to around −8°C) [12] . If true, this could explain the Mpemba effect because the initially cold water would need to "do more work"; — that is, get colder — in order to freeze. However, this also cannot be considered "the" sole explanation of the Mpemba effect.

First of all, as far as I know, this result has not been independently confirmed. The experiment described above [12] only had a limited number of trials, so the results found could have been a statistical fluke. Second, even if the results are true, they do not fully explain the Mpemba effect, but replace one mystery with another. Why should initially hot water supercool less than initially cold water? After all, once the water has cooled to the lower temperature, one would generally expect that the water would not "remember" what temperature it used to be. One explanation is that the initially hot water has less dissolved gas than the initially cold water, and that this affects its supercooling properties (see Dissolved Gasses for more on this). The problem with this explanation is that one would expect that since the hot water has less dissolved gas, and thus fewer nucleation sites, it would supercool more, not less. Another explanation is that when the initially hot water has cooled down to 0°C (or less), its temperature distribution throughout the container varies more than the initially cold water (see Convection for more on this).

Since temperature shear induces freezing [26] , the initially hot water supercools less, and thus freezes sooner. Third, this explanation cannot work in all of the experiments, because many of the experimenters chose to look not at the time to form a complete block of ice, but the time for some part of the water to reach 0°C [7,10,13] (or perhaps the time for a thin layer of frost to form on the top [17] ). While [12] says that it is only a "true Mpemba effect" if the hot water freezes entirely first, other papers have defined the Mpemba effect differently. Since the precise time of supercooling is inherently unpredictable (see e.g. [26] ), many experiments have chosen to measure not the time for the sample to actually become ice, but the time for which the sample's equilibrium ground state is ice; that is, the time when the top of the sample reached 0°C [7,10,13] . The supercooling argument does not apply to these experiments. References Historical 1. Aristotle in E. W. Webster, "Meteorologica I", Oxford U.

P., Oxford, 1923, pgs 348b–349a 2. Bacon F 1620 Novum Organum Vol VIII of "The Works of Francis Bacon" 1869 ed. J. Spedding, R. L. Ellis and D. D. Heath (New York) pp 235, 337, quoted in T.S. Kuhn 1970 "The Structure of Scientific Revolutions" 2nd edn (Chicago: University of Chicago Press), pg 16 3. Descartes R 1637, "Les Meteores" 164 published with "Discours de la Methode" (Leyden: Ian Marie) 1637, quoted in "Oeuvres de Descartes" Vol. VI 1902 ed. Adam and Tannery (Paris: Leopold Cerf) pg 238 (trans. F. C. Frank) 4.

Clagett, Marshall, "Giovanni Marliani and Late Medieval Physics", AMS press, Inc., New York, 1967, pgs 72, 79, 94 Experiments on the Mpemba Effect 5. Mpemba and Osborne, "Cool", Physics Education 4, pgs 172–5 (1969) 6. Ahtee, "Investigation into the Freezing of Liquids", Phys. Educ. 4, pgs 379–80 (1969) 7. I. Firth, "Cooler?", Phys. Educ. 6, pgs 32–41 (1979) 8. E. Deeson, "Cooler—lower down", Phys. Educ. 6, pgs 42–44 (1971) 9. Osborne, "Mind on Ice", Phys. Educ. 14, pgs 414–17 (1979) 10. M. Freeman, "Cooler Still", Phys. Educ.

14, pgs 417–21 (1979) 11. G.S. Kell, "The Freezing of Hot and Cold Water", American Journal of Physics, 37, #5, pgs 564–5 (May 1969) 12. D. Auerbach, "Supercooling and the Mpemba effect: When hot water freezes quicker than cold", American Journal of Physics, 63, #10, pgs 882–5 (Oct 1995) 13. J. Walker, "The Amateur Scientist", Scientific American, 237, #3, pgs 246–7 (Sept. 1971) 14. B. Wojciechowski, "Freezing of Aqueous Solutions Containing Gases", Cryst. Res.

Technol., 23, #7, pgs 843–8 (1988) We've Finally Figured Out Why Hot Water Freezes Faster Than Cold We've Finally Figured Out Why Hot Water Freezes Faster Than Cold For centuries, scientists have puzzled over a counter-intutive observation: hot water, for some reason, seems to freeze faster than cold. Fortunately, now a team of physicists has worked out why it happens . Advertisement Known as the Mpemba effect—after a Tanzanian student who noticed that hot ice cream mix freezes faster than a cold—it was in fact first observed by Aristotle, then later Francis Bacon and René Descartes. But while it's been observed, recorded, and discussed by eminent thinkers for years, nobody has ever worked out why hot water freezes more quickly than cold. Enter Xi Zhang and his colleagues from the Nanyang Technological University in Singapore. They've found evidence which suggests that it's the chemical bonds that hold water together which provide the strange effect. First, some chemistry. Each molecule of water is made up of two hydrogen atoms bonded covalently to a single atom of oxygen. Those bonds, which involve atoms sharing electrons, are well understood.

But the separate water molecules are bound together, too, by weaker forces generated by hydrogen bonds . They occur when a hydrogen atom from one molecule of water sits close to an oxygen atom from another—and they give rise to many of water's interesting properties, like its strangely high boiling point. Advertisement Now, Xi Zhang is suggesting that those same bonds cause the Mpemba effect. The idea is pretty simple: bring water molecules into close contact, and a natural repulsion between the molecules causes the covalent bonds to stretch and store energy. As the the liquid warms up, the hydrogen bonds stretch as the water gets less dense and the molecules move further apart. That extra stretch in hydrogen bonds allows the covalent bonds to relax and shrink a little, giving up their energy. The process of covalent bonds giving up energy is equivalent to cooling, so warm water should in theory cool faster than cold. Which is the Mpemba effect! Xi Zhang's theoretical calculations suggest that the magnitude of the covalent bond relaxation exactly accounts for the experimental differences in the time it takes for hot and cold water to freeze—which is, frankly, amazing. It's worth noting that the work isn't peer-reviewed yet , but it does seem to neatly explain the theory.

And considering nobody else has a decent explanation, it seems likely to be right. [ arXiv via Medium ] Sponsored Why Hot Water Freezes Faster Than Cold—Physicists Solve the Mpemba Effect An alternative view of the best new ideas in science. About: Oct 29, 2013 Why Hot Water Freezes Faster Than Cold—Physicists Solve the Mpemba Effect Aristotle first noticed that hot water freezes faster than cold, but chemists have always struggled to explain the paradox. Until now Water may be one of the most abundant compounds on Earth, but it is also one of more mysterious. For example, like most liquids it becomes denser as it cools. But unlike them, it reaches a state of maximum density at 4°C and then becomes less dense before it freezes. In solid form, it is less dense still, which is why standard ice floats on water. That’s one reason why life on Earth has flourished— if ice were denser than water, lakes and oceans would freeze from the bottom up, almost certainly preventing the kind of chemistry that makes life possible.

Then there is the strange Mpemba effect, named after a Tanzanian student who discovered that a hot ice cream mix freezes faster than a cold mix in cookery classes in the early 1960s. (In fact, the effect has been noted by many scientists throughout history including Aristotle, Francis Bacon and René Descartes.) The Mpemba effect is the observation that warm water freezes more quickly than cold water. The effect has been measured on many occasions with many explanations put forward. One idea is that warm containers make better thermal contact with a refrigerator and so conduct heat more efficiently. Hence the faster freezing. Another is that warm water evaporates rapidly and since this is an endothermic process, it cools the water making it freeze more quickly. None of these explanations are entirely convincing, which is why the true explanation is still up for grabs. Today Xi Zhang at the Nanyang Technological University in Singapore and a few pals provide one. These guys say that the Mpemba paradox is the result of the unique properties of the different bonds that hold water together. What’s so odd about the bonds in water? A single water molecule consists of a relatively large oxygen atom joined to two smaller hydrogen atoms by standard covalent bonds.

But put water molecules together and hydrogen bonds also begin to play an important role. These occur when a hydrogen in one molecule comes close the oxygen in another and bonds to it. Hydrogen bonds are weaker than covalent bonds but stronger than the van der Waals forces that geckos use to climb walls. Chemists have long known that they are important. For example, water’s boiling point is much higher than other liquids of similar molecules because hydrogen bonds hold it together. But in recent years, chemists have become increasingly aware of more subtle roles that hydrogen bonds can play. For example, water molecules inside narrow capillaries form into chains held together by hydrogen bonds. This plays an important role in trees and plants where water evaporation across a leaf membrane effectively pulls a chain of water molecules up from the roots. Now Xi and co say hydrogen bonds also explain the Mpemba effect. Their key idea is that hydrogen bonds bring water molecules into close contact and when this happens the natural repulsion between the molecules causes the covalent O-H bonds to stretch and store energy. But as the liquid warms up, it forces the hydrogen bonds to stretch and the water molecules sit further apart.

This allows the covalent molecules to shrink again and give up their energy. The important point is that this process in which the covalent bonds give up energy is equivalent to cooling. In fact, the effect is additional to the conventional process of cooling. So warm water ought to cool faster than cold water, they say. And that’s exactly what is observed in the Mpemba effect. These guys have calculated the magnitude of the additional cooling effect and show that it exactly accounts for the observed differences in experiments that measure the different cooling rates of hot and cold water. Voila! That’s an interesting insight into the complex and mysterious properties of water, which still give chemists sleepless nights. But while Xi and co’s idea is convincing, it is not quite the theoretical slam dunk that many physicists will require to settle the question. That’s because the new theory lacks predictive power—at least in this paper. Xi and co need to use their theory to predict a new property of water that conventional thinking about water does not. For example, the shortened covalent bonds might give rise to some measurable property of the water that would not otherwise be present. The discovery and measurement of this property would be the coup de grâce that their theory needs.

So while these guys may well have solved the riddle of Mpemba effect, they will probably need to work a little harder to convince everyone. Nevertheless, interesting stuff! Ref: arxiv.org/abs/1310.6514 : O:H-O Bond Anomalous Relaxation Resolving Mpemba Paradox This Week’s Top 5 Posts Why can hot water freeze faster than cold? – Starts With A Bang! – Medium BlockedUnblock FollowFollowing The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist. Jan 10, 2014 Why can hot water freeze faster than cold? It’s known as the Mpemba effect, and it’s been observed as far back as Aristotle. But hot water really can sometimes freeze faster than cold, and the science of why is only beginning to be fully understood. Image credit: Karen Stray Nolting of Princeton Landing News.

“I have lived in a flurry of images, but I will go out in a freeze frame.” -Anthony Quinn You’ve all seen the footage and heard of the trick by now: throw a pot of boiling water in the air and watch it turn into snow. (Assuming you don’t burn yourself instead .) The physics behind this weird behavior is incredibly interesting and related to what happens to water in space , but there’s an even more bizarre and counterintuitive phenomena out there, as my old college friend Ricardo asks: In some circumstances, warmer water can freeze faster than colder water. Why is this? This is known as the Mpemba effect , and believe it or not, it’s real. Image credit Kenn Wilson / flickr. The effect is named after a Tanzanian schoolchild, Erasto Mpemba, who noticed while making ice cream with his classmates that warm milk froze faster than cold milk. Although this type of effect has been observed many times historically, on the surface it hardly makes any sense. Let’s think about why. Image credit: A World of Weather: Fundamentals of Meteorology .

Normally, if you start with liquid water, you can either add energy to it, heating it to its boiling point of 100 °C (212 °F) and continuing to add energy as it goes through its phase change into water vapor, or you can remove energy from it, cooling down to its freezing point of 0 °C (32 °F) and continuing to remove heat as you turn it into ice. It would only make sense that if you began with colder water, it would freeze faster, since it would take less time to reach the freezing point in the first place! In fact, most experiments that you can do will show exactly this: the colder water freezes first. Image credit: picotech.com, retrieved from Douglas Clark at . Even a “failed” experiment like this, however, offers a clue to how hot water might freeze faster than cold: notice how much faster the hot water cools down than the cold!

Again, this is very intuitive, since if you put a 10 °C pot of water and a 90 °C pot of water in -10 °C surroundings, the one with the 100 °C temperature difference is going to lose heat a lot more quickly than the one with just a 20 °C difference. But there’s a little more to the story than that, and it has everything to do with the unique properties of water . Image credit: Simplebooklet, via #wpKey=m2w0ULHgGA4y0coQhrdUVI#page=1 . You see, water is a very polar molecule, with an extremely electronegative oxygen atom attached to two hydrogens. In chemistry-speak, each oxygen is an electron hog, meaning that portion of the molecule is generally negatively charged, while the hydrogen side is electron-deficient, leaving it positively charged. And if you take molecules that have negative-and-positive ends and put a whole bunch of them together, they’re going to form loose bonds with one another; this is known as hydrogen bonding . Image credit: Wikimedia commons user Benjah-bmm27 .

Now in the liquid phase, water molecules are free to rotate and move around some, much moreso than in the solid phase but not nearly as much as in the gaseous phase. But what do you think happens when you have hot water versus cold water? You’re probably familiar with this childhood experiment: dropping food coloring into both hot water and cold water. The hotter the water is, the faster the individual molecules can race around and disperse. What’s going on at a molecular level is that the hotter any substance is: the faster your molecules move around, the easier it is to spontaneously break those tenuous, intermolecular hydrogen bonds, and the more the covalent bonds in each molecule itself lengthen. At least, those three things are what typically happen for a liquid substance. But water is just a little bit unusual . Image credit: Brooklyn College, CUNY, via . The funny thing about it is that at cold (low) temperatures, each molecule of water typically has at least four neighboring water molecules, each tugging on this highly polar molecule. These neighboring molecules — even with their weak hydrogen bonds — effectively stretch the covalent bonds between the hydrogen and oxygen atoms. Image credit: user Qwerter at the Czech Wikipedia.

This roughly tetrahedral structure around each water molecule is highly disrupted in hotter water, meaning that there’s no longer this inter-molecular stretching going on. So while the water molecules are moving around faster, and it is easier to break these tenuous hydrogen bonds, the covalent bonds inside each water molecule actually shrink as they increase in temperature! So of the three standard things that typically happen for liquids, two happen for water, but the opposite happens for the third! So for hot water, these covalent bonds are shorter and stiffer, and when you cool it, contracting the hydrogen bonds, that forces the covalent bonds to lengthen, which means a faster relaxation time and — under the right conditions — a faster arrival at the freezing point for initially hotter water! Image credit: Xi Zhang Yongli Huang, Zengsheng Ma, and Chang Q Sun (2013), via . The higher temperature the water starts off at, the more energy is stored in those shorter, stiffer covalent bonds, and then placing that water in a very cold environment causes energy to be released at a rate that is exponentially dependent on the initial bond energy!

(Below to the left, you can see how τ, the energy release timescale, is much shorter at higher initial temperatures, and at right, you can see how the energy of the covalent bond is greater at higher initial temperatures.) Image credit: Xi Zhang Yongli Huang, Zengsheng Ma, and Chang Q Sun (2013), via . Experimentally, the best way to reproduce this result is to have relatively small amounts of cool water and water that’s nearly boiling as your initial subjects, and a “cold environment” that isn’t too much colder than freezing, but that’s large enough to be unaffected by the heat of the liquid water. And that is our current understanding of why the Mpemba effect happens, or why initially hotter water can freeze faster than cold! 4 The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist. Follow The Universe is out there, waiting for you to discover it.

4 The Claim: Cold Water Boils More Quickly Than Hot Water - The New York Times The New York Times Health |The Claim: Cold Water Boils More Quickly Than Hot Water Search Continue reading the main story THE FACTS The claim is repeated so often that many people accept it as fact. But according to scientists, the notion that a body of cold water will reach boiling temperature more quickly than an identical body of hot water under the same parameters is simply false. This kitchen myth may have started as a way to encourage people to cook with cold water, not hot, which can contain more impurities. It may also have its origins in the fact that cold water generally gains heat more rapidly than water that is already hot, though it will not boil faster. Photo Credit Leif Parsons But under the right circumstances, the reverse phenomenon can occur, and hot water can freeze more quickly than cool water. Part of the reason appears to be that hotter water loses mass to evaporation, and because it has less mass, less energy is needed to freeze it. That phenomenon was described as far back as 350 B.C. by Aristotle. The phenomenon is now known as the Mpemba effect, named after Erasto B.

Mpemba, a student who noticed it in 1963 while using boiled milk to make ice cream and reintroduced the concept to the scientific literature. A study published in the American Journal of Physics in 2006 described a number of reasons for the phenomenon. THE BOTTOM LINE Cold water does not boil faster than hot water, but hot water can freeze more quickly than cool water. Why does hot water freeze faster than cold water? - ScienceAlert Why does hot water freeze faster than cold water? Yep, it's true - and the chemistry is fascinating. SIGNE DEAN 2 JAN 2016 Lots of you in the Northern Hemisphere will be in the middle of yet another winter, and some might even be experiencing the sub-zero temperatures to do some cool experiments, such as creating giant frozen marbles for the front yard , snapping soap bubbles , or even tossing boiling water into air to create snow (although that last one requires caution, seriously). If you remember the dreadfully cold winter of 2013, you might have seen videos like the one we've posted below circulating as 'proof' of the crazy low temperatures.

But it turns out this demonstration is actually not that surprising - hot water is in fact known to freeze faster than cold water. There are records of this from as far back as Aristotle's time. In modern times, this counterintuitive property has been named the Mpemba effect, after a Tanzanian secondary school student who re-discovered this phenomenon back in 1963. Erasto Mpemba and other kids at his school often made ice cream using the school freezer - they would do this by boiling milk and mixing it with sugar, which then had to be cooled and placed in the freezer. One day, Mpemba rushed the process and stuck the milk in while still hot, and to his surprise, the ice cream formed quicker than for his classmate. Unsurprisingly, none of his teachers believed the 13-year-old. Mpemba later teamed up with a physics professor who visited his high school, and in 1969 they published a paper , which has since been replicated many times - most often with a similar result.

Although if you try this at home and fail, it's probably because the Mpemba effect is not a reliable phenomenon that happens every single time; in fact, there seem to be several factors at play. So what are they? Well, it's one of those somewhat unsatisfying cases where just because we know something happens, doesn't mean we entirely understand why. But the process of scientific speculation is still interesting, so here are some versions. The most commonly proposed hypothesis - and one that's probably somewhat responsible for the effect - is that hot water evaporates more quickly, losing mass and therefore needing to lose less heat in order to freeze. However, scientists have also demonstrated the Mpemba effect with closed containers where evaporation doesn't take place. Another theoretical speculation is that water develops convection currents and temperature gradients as it cools - a rapidly cooling glass of hot water will have greater temperature differences throughout, and lose heat more quickly from the surface, whereas a uniformly cool glass of water has less of a temperature difference, and there's less convection to accelerate the process. But this idea has not been entirely verified either.

Other theories have been put forward, including supercooling, or the effect that dissolved gasses in the water would have on the freezing process. It’s likely that several of these actually come into play. In late 2013, a team of researchers from Singapore proposed in a paper on arXiv.org that the Mpemba paradox stems from the unique properties of chemical bonds in the water. A standard water molecule contains one oxygen atom and two hydrogen atoms joined to it by covalent bonds sharing electron pairs between the atoms. But when you put several water molecules together, the hydrogen atoms will also form bonds with oxygen atoms in other molecules. These hydrogen bonds are what gives water some of its properties, such as having a relatively high boiling point, and becoming less dense when frozen. According to the researchers, when water boils, the molecules spread out, lengthening the hydrogen bonds - but because volume is limited, meanwhile the covalent bonds within individual molecules get compressed, storing away energy. If water is frozen at this state, the bonds release energy as an uncoiled spring, cooling down much more quickly than if the covalent bonds were less compressed.

However, even though many headlines heralded this as the definitive explanation for the Mpemba effect, the idea is yet to be published in a peer-reviewed journal. According to the blog of Physics arXiv , the theory is convincing, but lacks predictive power: "Xi and co need to use their theory to predict a new property of water that conventional thinking about water does not. For example, the shortened covalent bonds might give rise to some measurable property of the water that would not otherwise be present. The discovery and measurement of this property would be the coup de grâce that their theory needs." As Adam Mann wrote at Wired , this explanation is also not strongly supported in the scientific community. "When we talked to chemist Richard Zare of Stanford University, he was unconvinced of its merits and said he thinks the dominant force at play is evaporation." Still, it’s fun to (carefully) try this at home. More From ScienceAlert Mpemba effect Grossly misleading paper Hot water may freeze faster than cold water The ability of hot water to freeze faster than cold seems counter-intuitive as it would seem that hot water must first become cold water and therefore the time required for this will always delay its freezing relative to cold water.

However experiments show that hot water (for example, 90 °C) does often (but by no means always) appear to freeze faster than the same amount of cold water (for example, 18 °C) under otherwise identical conditions [ 158 ] (see also graph below left [ 2810 ] ). This has been recognized even as far back as Aristotle in the 4th century BC [ 2043 ] but was brought to the attention of the scientific community by the perseverance of Erasto Mpemba [ 1388 ] a Tanzanian schoolboy, a who refused to reject his own evidence, or bow to disbelieving mockery, that he could freeze ice cream faster if he warmed it first. Unfortunately the Mpemba effect (Hot water may freeze faster than cold water) is not well formulated for precise testing as the temperatures are not stated nor is the amount of freezing. It was never meant to be precisely determinable, as are other anomalies such as the temperature of maximum density, but more as an interesting natural phenomenon that has been noticed since the beginning of recorded history and contains an interesting and edifying story for students.

No standard experimental setup is described with various cooling regimes being used causing differences in observations and explanations depending on, for example, the base cooling temperature. Generally however, it is accepted that typical difference in start temperatures may be about 40°C and the extent of freezing is that it should be visibly extensive but not complete. In the definition, the word 'may' is fairly interpreted as 'more often than not', and certainly not as 'always'. A number of explanations have been put forward [ 959 , 1921 ]. b One that has gained some support is that there is sufficient evaporation from the hot water that this causes faster cooling plus a reduction in mass, so faster cooling (even to the extent of the existence of a cross-over temperature between, initially hot and initially colder, cooling curves [ 1716 ]) and freezing [ 1390 ]. There does not seem to be sufficient mass lost in experiments to support such an explanation as the sole cause, however, and the Mpemba effect is apparent even when the vessel has a lid. Related to this explanation, part of the cause may be due to the variously sized and persistent convective currents created when hot water cools in different experimental set-ups.

The importance of gas content has been shown to be involved with high statistical certainty [ 2810 ] . Another scenario concerns the O:H–O hydrogen bond possessing memory, whose thermal relaxation defines intrinsically the rate of energy emission [ 2227 ]. This hypothesis, although innovative, needs further substantiation as individual bonds between neighboring oxygen atoms (of H2O molecules) have only fleeting lifetimes, d this explanation describes an effect, which is apparently insensitive to many factors known to have effects such as gas content, convection and evaporation, and the described cooling curves have recently been challenged [ 2809 ]. The initial amount of ice produced on nucleation is about 1.3% for every degree of supercooling. The most likely scenario (described in [ 158 ], disputed [ 1415 ], but later supported [ 2047 ]) is that the degree of supercooling is greater, under some circumstances, in initially-cold water than initially-hot water. The initially-hot water appears to freeze at a higher temperature (less supercooling) but less of the apparently frozen ice is solid and a considerable amount is trapped liquid water.

Initially-cold water freezes at a lower temperature to a more completely solid ice with less included liquid water; the lower temperature causing intensive nucleation and a faster crystal growth rate. If the freezing temperature is kept about -6 °C then the initially-hot water is most likely to (apparently) freeze first. If freezing is continued, initially-cold water always completely freezes before initially-hot water. After coming to this conclusion, it was found that this explanation for the Mpemba effect was first experimentally determined in 1775 [ 1861 ]. Many factors, including the temperature of the cooling bath, the material in the cooling bath and the physical dimensions and characteristics of the vessels all affect both the rate of cooling and whether, or not, supercooling occurs [ 2281 ] such that these factors must be clearly stated and controlled. Such factors must be taken into account when comparing different experimental procedures. Why initially-cold water supercools more is explained in terms of the gas concentration and the clustering of water. Certainly, water behaves differently, and possesses a different structuring, at the same temperature depending upon whether it is being heated or cooled [ 1697 ].

Icosahedral clusters do not readily allow the necessary arrangement of water molecules to enable hexagonal ice crystal initiation; such clustering is the cause of the facile supercooling of water . Water that is initially-cold will have the maximum (equilibrium) concentration of such icosahedral clustering. Initially-hot water has lost much of its ordered clustering and, if the cooling time is sufficiently short, this will not be fully re-attained before freezing. Experiments on low-density water around macromolecules have shown that such clustering processes may take some time [ 4 ]. Also of relevance here is that the formation of clathrate ices , which have structures closely related to the icosahedral clusters, behaves in an opposite manner. Thus, their supercooling (before clathrate ice formation) from hot water is far greater than that from cold water [ 1391 ]. c Recently, it has been discovered that the charge on the liquid interface affects the freezing point of supercooled water [ 1737 ]. As the surface of nanobubbles is thought to be negative, the presence of such nanobubbles, with their extensive surface area are expected to increase supercooling.

Heating water containing nanobubbles is expected to destroy nanobubbles as they grow in size, due to the lower gas solubility at higher temperatures, and dissipate. The Mpemba effect is then simply explained by the loss of nanobubbles in the hot water which are kinetically too slow to reform on cooling. It is also possible that dissolved gases may encourage supercooling ([ 2810 ]; mouse over figure above to see the effect of removing the gasses) by (1) increasing the degree of structuring, by hydrophobic hydration, in the previously-cold water relative to the gas-reduced previously-hot water (the critical effect of low concentrations of dissolved gas on water structure is reported in [ 294 ]; re-equilibration taking several days) and (2) increasing the pressure as gas comes out of solution when the water starts to crystallize, so lowering the melting point and reducing the tendency to freeze ( see guest book ). Also, the presence of tiny gas bubbles (cavities produced on heating), with their more extensive liquid-gas interface, may increase the rate of nucleation, so reducing supercooling [ 428 ].

Recently another possibility has been described depending on changes in dissolved material with temperature (such as the reduction in bicarbonate in heated 'hard' water), but this has not yet been experimentally tested [ 1014 ]. The rationale for the Mpemba effect in this case concerns differences in the solute concentration at the ice-liquid interface causing a localized lowering of the melting point [ 1014 ]. Grossly misleading paper Rarely do I take the trouble to criticize scientific publications but a paper that recently appeared in Nature Scientific Reports [ 2809 ], together with its BBC radio commentary, are highly misleading and require response. e The Mpemba effect concerns supercooling and the formation of ice whereas this paper only involves cooling to 0°C or 4°C but (importantly) avoids supercooling and ice formation and so, in spite of its title, is not concerned experimentally with the Mpemba effect but with the simple cooling of liquid water. The paper does show that cold water cools to 0°C faster than hot water and this seems to support the view given above that the Mpemba effect is down to differences in the supercooling of previously hot and cold liquid water.

The paper [ 2809 ] neither proves nor disproves the Mpemba effect;' Hot water may freeze faster than cold water'. e ] Footnotes a Erasto Mpemba is now enjoying his retirement from being Principal Game Officer in Tanzania. He describes his discovery on YouTube . [ Back ] b The winning entry in the Royal Society of Chemistry competition concerning the Mpemba effect is given here . [ Back ] c The gas-hydrate clathrate ices also show an anomalously low dissociation rate [ 1539 ] that fits well with this hypothesis. Later work has shown ambiguous results [ 2817 ]. [ Back ] d The O···H-O hydrogen bond has a lifetime of ~ns and the O···H-O covalent bond has a lifetime of ~ms. Assoc Prof Sun Changqing has strongly disagreed with my assessment of this area of science stating (rather obtusely) 'Fleeting times have nothing to do with the relaxation time for the energy "emission-transportation-dissipation" in the "source-path-drain" cycle system at all. ', Please see his papers [ 2227 ] for further details of his work.

[ Back ] e Burridge and Linden’s paper [ 2809 ] misstates what is known as the Mpemba effect. In their Abstract the authors state that the Mpemba effect is “the assertion that it is quicker to cool water to a given temperature when the initial temperature is higher”. However the early paper by Mpemba and Osborne [ 1388 ] states “If two systems are cooled, the water that starts hotter may freeze first”. Follow up papers (for example [ 158 , 959 , 1921 , 2810 ]) always have included the process of freezing within the Mpemba effect. In the Royal Society of Chemistry ’s competition inviting discussion of the effect in 2012, the Mpemba effect was defined as “The Mpemba effect is the phenomenon where hot water freezes quicker than cold water” and the competition received 22,000 entrants. A similar invitation from the Journal 'Temperature' (ISSN: 2332-8940) gave its definition as "warm water freezes more quickly than cold water" [ 959c ].

All previous discussions of the Mpemba effect have involved a visible formation of ice and include the observation that hot water initially freezes faster than cold water, at least some of the time. Burridge and Linden’s paper takes the view that “Broadly speaking, when two samples of water are cooled to the same temperature, in the same manner with the two samples being identical except for their initial temperature, and the initially hotter sample cools in less time, one can consider the Mpemba effect to have been observed” This mis-definition of the Mpemba effect considers cooling to a low temperature to be (mistakenly) the same as cooling to, and including visible ice formation. Their paper goes on to present experimental results, but only for the cooling process to temperatures well above those of freezing water and certainly not including the freezing process which was never observed. Burridge and Linden go on to dismiss all prior studies that included supercooling and freezing with a blanket statement that they are not meaningful. In so doing they dismiss the observations of many scientists including G. S. Kell [ 1390 ], without their own experimental evidence and with no support from any prior experiments. Of the Vynnycky and.

Kimura study [ 158 ], they state “exhibited no evidence of the Mpemba effect” but the paper actually states the contrary in its Abstract “the effect of supercooling leads to a spread in the experimental freezing times, giving results that constitute evidence of the occurrence of the Mpemba effect”. Burridge and Linden disregard data in this paper as it “cannot be fairly included in our analysis, since we exclude the freezing process”. It is unsurprising that Burridge and Linden state “We conclude that despite our best efforts, we were not able to make observations of any physical effects which could reasonably be described as the Mpemba effect” as they never performed any experiments nor made any associated observations in any attempt to do so. It is also clear from their paper that Burridge and Linden misunderstand the correct and widely-understood definition of the Mpemba effect There have been many attempts to explain the Mpemba effect but supercooling has often been cited. 30 years ago Wojciechowski et al [ 2810 ] concluded that dissolved gasses caused this supercooling anomaly with persuasive statistically-meaningful (P < 0.001) experimental results that Burridge and Linden failed to find.

To conclude, Burridge and Linden’s paper neither proves nor disproves the Mpemba effect and has a misleading Title and Abstract. Burridge and Linden's Reply Burridge has replied to my points and I give his unedited reply below even though I believe it to be mistaken, misleading and highly contentious. It also misstates comments I have made concerning ice formation. 'Thank you for sending these papers. I met with Professor Paul Linden, who as my co-author is well aware of all of our communications, and we discussed your opinions and the specific papers which you consider to be evidence. We regard the results of the 2015 paper published by the group at KTH as very strong evidence that our conclusions are entirely valid. These experiments were carried out carefully, were well documented and used current technology to improve precision - they show evidence that the Mpemba effect can be observed but also strong evidence that the Mpemba effect cannot be observed in a repeatable meaningful way paper - this is precisely what we concluded. With regards to the paper from the 1980's, I am disappointed that I had not found this paper prior to publishing our article - I would have liked to include reference to it, and comment upon it.

However, Paul and I agree that it would not have changed our conclusions nor the degree of certainty with which they hold true. The data included is interesting. However, like much of the work published on the Mpemba effect, it is fundamentally flawed. The paper does not clearly define nor describe details underpinning the matter of primary concern, i.e. how the to time for freezing to first occur was robustly determined. There are many issues concerning the formation of ice crystals at the smallest scale which are not discussed, for example, as you pointed out they can often melt back into the liquid state before ultimately freezing again. These, and numerous other experimental issues, result in observations of the time-for-freezing-to-first-occur being notoriously difficult to make in a robust and repeatable manner. Without discussion of all of the experimental difficulties, mitigating procedures, and evidence as to the extent of their precise effects, it is impossible to know if these results are statistically significant or not. As such, we still feel entirely able to stand by the conclusions of our paper.

However, if you are able to repeat this data with documented evidence of all mitigating procedures, error bounds and uncertainties then we would be delighted to review this data and once again consider whether our conclusions remain valid. In the absence of this data, I will cease to respond to any communications, since without this data our communications have ceased to be fruitful.'

Question: The fact that hot water freezes more quickly than cold water is known as what?
Answer: The Mpemba effect