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Is it true that hot water freezes faster than cold water?
Hot water does not freeze faster than cold water. Nor does cold water boil faster than hot water. For water to freeze, you must remove enough heat to bring its temperature down to 0 C. The colder the water is, the less heat you have to remove before it freezes. Answer I have no idea why this rumor is cirulating the net and the world. It is just totaly insane to even think that that could be true. If you have just heated the water to 100 degrese, then it has that much more to cool down to 0. If the waterisat 30 then it is obvious that the 30 degree would cool faster. Really now. Answer It's not exactly a rumor, it's true...at least sometimes. In scientific communities it's called "The Mpemba Effect" and was named after a Tanzanian student named Erasto Mpemba who first raised the question in the late 60s. When a container of water is placed in a freezer, there are many factors which can affect the length of time it will take to freeze, and under the proper combination of circumstances, a given volume of hot water in a container may freeze faster than the same volume of cold water. So, the best answer is, "Sometimes."
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Can hot water freeze faster than cold water? It depends. Hot watercan in fact freeze faster than cold water for a wide range ofexperimental conditions. This phenomenon is extr…emely counter-intuitive, and surprising even to most scientists, but it is infact real. It has been seen and studied in numerous experiments.While this phenomenon has been known for centuries, and wasdescribed by Aristotle, Bacon, and Descartes [1-3], it was notintroduced to the modern scientific community until 1969, by aTanzanian high school student named Mpemba. Both the earlyscientific history of this effect, and the story of Mpemba'srediscovery of it, are interesting in their own right -- Mpemba'sstory in particular provides a dramatic parable against making snapjudgments about what is impossible. This is described separatelybelow. The phenomenon that hot water may freeze faster than cold is oftencalled the Mpemba effect. Because, no doubt, most readers areextremely skeptical at this point, we should begin by statingprecisely what we mean by the Mpemba effect. We start with twocontainers of water, which are identical in shape, and which holdidentical amounts of water. The only difference between the two isthat the water in one is at a higher (uniform) temperature than thewater in the other. Now we cool both containers, using the exactsame cooling process for each container. Under some conditions theinitially warmer water will freeze first. If this occurs, we haveseen the Mpemba effect. Of course, the initially warmer water willnot freeze before the initially cooler water for all initialconditions. If the hot water starts at 99.9Â°C, and the cold waterat 0.01Â°C, then clearly under those circumstances, the initiallycooler water will freeze first. However, under some conditions theinitially warmer water will freeze first -- if that happens, youhave seen the Mpemba effect. But you will not see the Mpemba effectfor just any initial temperatures, container shapes, or coolingconditions. This seems impossible, right? Many sharp readers may have alreadycome up with a common proof that the Mpemba effect is impossible.The proof usually goes something like this. Say that the initiallycooler water starts at 30Â°C and takes 10 minutes to freeze, whilethe initially warmer water starts out at 70Â°C. Now the initiallywarmer water has to spend some time cooling to get to get down to30Â°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 theinitially cooler water has to do, plus a little more, it will takeat least a little longer, right? What can be wrong with this proof? What's wrong with this proof is that it implicitly assumes that thewater is characterized solely by a single number -- the averagetemperature. But if other factors besides the average temperatureare important, then when the initially warmer water has cooled toan average temperature of 30Â°C, it may look very different than theinitially cooler water (at a uniform 30Â°C) did at the start. Why?Because the water may have changed when it cooled down from auniform 70Â°C to an average 30Â°C. It could have less mass, lessdissolved gas, or convection currents producing a non-uniformtemperature distribution. Or it could have changed the environmentaround the container in the refrigerator. All four of these changesare conceivably important, and each will be considered separatelybelow. So the impossibility proof given above doesn't work. And infact the Mpemba effect has been observed in a number of controlledexperiments [5,7-14] It is still not known exactly why this happens. A number ofpossible explanations for the effect have been proposed, but so farthe experiments do not show clearly which, if any, of the proposedmechanisms is the most important one. While you will often hearconfident claims that X is the cause of the Mpemba effect, suchclaims are usually based on guesswork, or on looking at theevidence in only a few papers and ignoring the rest. Of course,there is nothing wrong with informed theoretical guesswork or beingselective in which experimental results you trust -- the problem isthat different people make different claims as to what X is. Why hasn't modern science answered this seemingly simple questionabout cooling water? The main problem is that the time it takeswater to freeze is highly sensitive to a number of details in theexperimental set- up, such as the shape and size of the container,the shape and size of the refrigeration unit, the gas and impuritycontent of the water, how the time of freezing is defined, and soon. Because of this sensitivity, while experiments have generallyagreed that the Mpemba effect occurs, they disagree over theconditions under which it occurs, and thus about why it occurs. AsFirth  wrote "There is a wealth of experimental variation in theproblem so that any laboratory undertaking such investigations isguaranteed different results from all others." So with the limited number of experiments done, often under verydifferent conditions, none of the proposed mechanisms can beconfidently proclaimed as "the" mechanism. Above we described fourways in which the initially warmer water could have changed uponcooling to the initial temperature of the initially cooler water.What follows below is a short description of the four relatedmechanisms that have been suggested to explain the Mpemba effect.More ambitious readers can follow the links to more completeexplanations of the mechanisms, as well as counter- arguments andexperiments that the mechanisms cannot explain. It seems likelythat there is no one mechanism that explains the Mpemba effect forall circumstances, but that different mechanisms are importantunder different conditions. . Evaporation -- As the initially warmer water cools to theinitial temperature of the initially cooler water, it may losesignificant amounts of water to evaporation. The reduced mass willmake it easier for the water to cool and freeze. Then the initiallywarmer water can freeze before the initially cooler water, but willmake less ice. Theoretical calculations have shown that evaporationcan explain the Mpemba effect if you assume that the water losesheat solely through evaporation . This explanation is solid,intuitive, and evaporation is undoubtedly important in mostsituations. However, it is not the only mechanism. Evaporationcannot explain experiments that were done in closed containers,where no mass was lost to evaporation . And many scientistshave claimed that evaporation alone is insufficient to explaintheir results [5,9,12]. . Dissolved Gasses -- Hot water can hold less dissolved gas thancold water, and large amounts of gas escape upon boiling. So theinitially warmer water may have less dissolved gas than theinitially cooler water. It has been speculated that this changesthe properties of the water in some way, perhaps making it easierto develop convection currents (and thus making it easier to cool),or decreasing the amount of heat required to freeze a unit mass ofwater, or changing the boiling point. There are some experimentsthat favor this explanation [10,14], but no supporting theoreticalcalculations. . Convection -- As the water cools it will eventually developconvection currents and a non-uniform temperature distribution. Atmost 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 heatprimarily through the surface, then water with a "hot top" willlose heat faster than we would expect based on its averagetemperature. When the initially warmer water has cooled to anaverage temperature the same as the initial temperature of theinitially cooler water, it will have a "hot top", and thus its rateof cooling will be faster than the rate of cooling of the initiallycooler water at the same average temperature. Got all that? Youmight want to read this paragraph again, paying careful distinctionto the difference between initial temperature, average temperature,and temperature. While experiments have seen the "hot top", andrelated convection currents, it is unknown whether convection canby itself explain the Mpemba effect. . Surroundings -- A final difference between the cooling of thetwo containers relates not to the water itself, but to thesurrounding environment. The initially warmer water may change theenvironment around it in some complex fashion, and thus affect thecooling process. For example, if the container is sitting on alayer of frost which conducts heat poorly, the hot water may meltthat layer of frost, and thus establish a better cooling system inthe long run. Obviously explanations like this are not verygeneral, since most experiments are not done with containerssitting on layers of frost. Finally, supercooling may be important to the effect. Supercoolingoccurs when the water freezes not at 0Â°C, but at some lowertemperature. One experiment  found that the initially hot waterwould supercool less than the initially cold water. This would meanthat the initially warmer water might freeze first because it wouldfreeze at a higher temperature than the initially cooler water. Iftrue, this would not fully explain the Mpemba effect, because wewould still need to explain why initially warmer water supercoolsless than initially cooler water. In short, hot water does freeze sooner than cold water under a widerange of circumstances. It is not impossible, and has been seen tooccur in a number of experiments. However, despite claims oftenmade by one source or another, there is no well-agreed explanationfor how this phenomenon occurs. Different mechanisms have beenproposed, but the experimental evidence is inconclusive. For thosewishing to read more on the subject, Jearl Walker's article inScientific American  is very readable and has suggestions onhow to do home experiments on the Mpemba effect, while the articlesby Auerbach  and Wojciechowski  are two of the more modernpapers on the effect. History of the Mpemba Effect The fact thathot water freezes faster than cold has been known for manycenturies. The earliest reference to this phenomenon dates back toAristotle in 300 B.C. The phenomenon was later discussed in themedieval era, as European physicists struggled to come up with atheory of heat. But by the 20th century the phenomenon was onlyknown as common folklore, until it was reintroduced to thescientific community in 1969 by Mpemba, a Tanzanian high schoolstudent. Since then, numerous experiments have confirmed theexistence of the "Mpemba effect", but have not settled on anysingle explanation. The earliest known reference to this phenomenon is by Aristotle,who wrote: "The fact that water has previously been warmed contributes toits freezing quickly; for so it cools sooner. Hence many people,when they want to cool hot water quickly, begin by putting it inthe sun. . ." [1,4] He wrote these words in support of a mistaken idea which he calledantiperistasis. Antiperistasis is defined as "the supposed increasein the intensity of a quality as a result of being surrounded byits contrary quality, for instance, the sudden heating of a warmbody when surrounded by cold" . Medieval scientists believed in Aristotle's theory ofantiperistasis, and also sought to explain it. Not surprisingly,scientists in the 1400's had trouble explaining how it worked, andcould not even decide whether (as Aristotle claimed in support ofantiperistasis), human bodies and bodies of water were hotter inthe winter than in the summer . Around 1461, the physicistGiovanni Marliani, in a debate over how objects cooled, said thathe had confirmed that hot water froze faster than cold. He saidthat he had taken four ounces of boiling water, and four ounces ofnon-heated water, placed them outside in similar containers on acold winter day, and observed that the boiled water froze first.Marliani was, however, unable to explain this occurrence . Later, in the 1600's, it was apparently common knowledge that hotwater would freeze faster than cold. In 1620 Bacon wrote "Waterslightly warm is more easily frozen than quite cold" , while alittle later Descartes claimed "Experience shows that water thathas been kept for a long time on the fire freezes sooner than otherwater" . In time, a modern theory of heat was developed, and the earlierobservations of Aristotle, Marliani, and others were forgotten,perhaps because they seemed so contradictory to modern concepts ofheat. However, it was still known as folklore among manynon-scientists in Canada , England [15-21], the food processingindustry , and elsewhere. It was not reintroduced to the scientific community until 1969, 500years after Marliani's experiment, and more than two millenniaafter Aristotle's "Meteorologica I" . The story of itsrediscovery by a Tanzanian high school student named Mpemba iswritten up in the New Scientist . The story provides a dramaticparable cautioning scientists and teachers against dismissing theobservations of non-scientists and against making quick judgmentsabout what is impossible. In 1963, Mpemba was making ice cream at school, which he did bymixing boiling milk with sugar. He was supposed to wait for themilk to cool before placing it the refrigerator, but in a rush toget scarce refrigerator space, put his milk in without cooling it.To his surprise, he found that his hot milk froze into ice creambefore that of other students. He asked his physics teacher for anexplanation, but was told that he must have been confused, sincehis observation was impossible. Mpemba believed his teacher at the time. But later that year he meta friend of his who made and sold ice cream in Tanga town. Hisfriend told Mpemba that when making ice cream, he put the hotliquids in the refrigerator to make them freeze faster. Mpembafound 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 certainsimplifying assumptions). Mpemba asked his teacher why hot milkfroze before cold milk when he put them in the freezer. The teacheranswered that Mpemba must have been confused. When Mpemba keptarguing, the teacher said "All I can say is that is Mpemba'sphysics and not the universal physics" and from then on, theteacher and the class would criticize Mpemba's mistakes inmathematics and physics by saying "That is Mpemba's mathematics" or"That is Mpemba's physics." But when Mpemba later tried theexperiment with hot and cold water in the biology laboratory of hisschool, he again found that the hot water froze sooner. Earlier, Dr Osborne, a professor of physics, had visited Mpemba'shigh school. Mpemba had asked him to explain why hot water wouldfreeze before cold water. Dr Osborne said that he could not thinkof any explanation, but would try the experiment later. When backin his laboratory, he asked a young technician to test Mpemba'sclaim. The technician later reported that the hot water frozefirst, and said "But we'll keep on repeating the experiment untilwe get the right result." However, repeated tests gave the sameresult, and in 1969 Mpemba and Osborne wrote up their results . In the same year, in one of the coincidences so common in science,Dr Kell independently wrote a paper on hot water freezing soonerthan cold water. Kell showed that if one assumed that the watercooled primarily by evaporation, and maintained a uniformtemperature, the hot water would lose enough mass to freeze first. Kell thus argued that the phenomenon (then a common urbanlegend in Canada) was real and could be explained by evaporation.However, he was unaware of Osborne's experiments, which hadmeasured the mass lost to evaporation and found it insufficient toexplain the effect. Subsequent experiments were done with water ina closed container, eliminating the effects of evaporation, andstill found that the hot water froze first . Subsequent discussion of the effect has been inconclusive. Whilequite a few experiments have replicated the effect [4,6-13], therehas been no consensus on what causes the effect. The differentpossible explanations are discussed above. The effect hasrepeatedly a topic of heated discussion in the "New Scientist", apopular science magazine. The letters have revealed that the effectwas known by laypeople around the world long before 1969. Today,there is still no well-agreed explanation of the Mpemba effect.More-detailed explanations Evaporation One explanation of theeffect is that as the hot water cools, it loses mass toevaporation. With less mass, the liquid has to lose less heat tocool, and so it cools faster. With this explanation, the hot waterfreezes first, but only because there's less of it to freeze.Calculations done by Kell in 1969  showed that if the watercooled solely by evaporation, and maintained a uniform temperature,the warmer water would freeze before the cooler water. This explanation is solid, intuitive, and undoubtedly contributesto the Mpemba effect in most physical situations. However, manypeople have incorrectly assumed that it is therefore "the"explanation for the Mpemba effect. That is, they assume that theonly reason hot water can freeze faster than cold is because ofevaporation, and that all experimental results can be explained bythe calculations in Kell's article. However, the experimentscurrently do not bear out this belief. While experiments showevaporation to be important , they do not show that it is theonly mechanism behind the Mpemba effect. A number of experimentershave argued that evaporation alone is insufficient to explain theirresults [5,9,12] -- in particular, the original experiment byMpemba and Osborne measured the mass lost to evaporation, and foundit substantially less that the amount predicted by Kell'scalculations [5,9]. And most convincingly, an experiment byWojciechowski observed the Mpemba effect in a closed container,where no mass was lost to evaporation. Dissolved Gasses Anotherexplanation argues that the dissolved gas usually present in wateris expelled from the initially hot water, and that this changes theproperties of the water in some way that explains the effect. Ithas been argued that the lack of dissolved gas may change theability of the water to conduct heat, or change the amount of heatneeded to freeze a unit mass of water, or change the freezing pointof the water by some significant amount. It is certainly true thathot water holds less dissolved gas than cold water, and that boiledwater expels most dissolved gas. The question is whether this cansignificantly affect the properties of water in a way that explainsthe Mpemba effect. As far as I know, there is no theoretical worksupporting this explanation for the Mpemba effect. Indirect support can be found in two experiments that saw theMpemba effect in normal water which held dissolved gasses, butfailed to see it when using degassed water [10,14]. However, anattempt to measure the dependence of the enthalpy of freezing onthe initial temperature and gas content of the water wasinconclusive . One problem with this explanation is that many experimentspre-boiled both the initially hot and initially cold water,precisely to eliminate the effect of dissolved gasses, and yet theystill saw the effect [5,13]. Two somewhat unsystematic experimentsfound that varying the gas content of the water made no substantialdifference to the Mpemba effect [9,12]. Convection It has also beenproposed that the Mpemba effect can be explained by the fact thatthe temperature of the water becomes non-uniform. As the watercools, temperature gradients and convection currents will develop.For most temperatures, the density of water decreases as thetemperature increases. So over time, as water cools we will developa "hot top" -- the surface of the water will be warmer than theaverage temperature of the water, or the water at the bottom of thecontainer. If the water loses heat primarily through the surface,then this means that the water should lose heat faster than onewould expect based just on looking at the average temperature ofthe water. And for a given average temperature, the heat lossshould be greater the more inhomogenous the temperaturedistribution is (that is, the greater the range of the temperaturesseen as we go from the top to the bottom). How does this explain the Mpemba effect? Well, the initially hotwater will cool rapidly, and quickly develop convection currentsand so the temperature of the water will vary greatly from the topof the water to the bottom. On the other hand, the initially coolwater will have a slower rate of cooling, and will thus be slowerto develop significant convection currents. Thus, if we compare theinitially hot water and initially cold water at the same averagetemperature, it seems reasonable to believe that the initially hotwater will have greater convection currents, and thus have a fasterrate of cooling. To consider a concrete example, suppose that theinitially hot water starts at 70Â°C, and the initially cold waterstarts at 30Â°C. When the initially cold water is at an average30Â°C, it is also a uniform 30Â°C. However, when the initially hotwater reaches an average 30Â°C, the surface of the water is probablymuch warmer than 30Â°C, and it will thus lose heat faster than theinitially cold water for the same average temperature. Got that?This explanation is pretty confusing, so you might want to go backand read the last two paragraphs again, paying careful attention tothe difference between initial temperature, average temperature,and surface temperature. At any rate, if the above argument is right, then when we plot theaverage temperature versus time for both the initially hot andinitially cold water, then for some average temperatures theinitially hot water will be cooling faster than the initially coldwater. So the cooling curve of the initially hot water will notsimply reproduce the cooling curve of the initially cold water, butwill drop faster when in the same temperature range. This shows that the initially hot water goes faster, but of courseit also has farther to go. So whether it actually finishes first(that is, reaches 0Â°C first), is not clear from the abovediscussion. To know which one finishes first would requiretheoretical modeling of the convection currents (hopefully for arange of container shapes and sizes), which has not been done. Soconvection alone may be able to explain the Mpemba effect, butwhether it actually does is not currently known. Experiments on theMpemba effect have often reported a "hot top" [5,8,10], as we wouldexpect. Experiments have been done that looked at the convectioncurrents of freezing water [27,28], but their implications for theMpemba effect are not entirely clear. It should also be noted that the density of water reaches a maximumat fourÂ° C. So below fourÂ°C, the density of water actuallydecreases with decreasing temperature, and we will get a "coldtop." This makes the situation even more complicated. SurroundingsThe initially hot water may change the environment around it insome way that makes it cool faster later on. One experimentreported significant changes in the data simply upon changing thesize of the freezer that the container sat in . So conceivablyit 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 thecontainer is resting on a thin layer of frost, than the containerholding the cold water will simply sit on the surface of the frost,while the container with the hot water will melt the frost, andthen be sitting on the bottom of the freezer. The hot water willthen have better thermal contact with the cooling systems. If themelted frost refreezes into an ice bridge between the freezer andthe container, the thermal contact may be even better. Obviously, even if this argument is true, it has fairly limitedutility, since most scientific experiments are careful enough notto rest the container on a layer of frost in a freezer, but insteadplace the container on a thermal insulator, or in a cooling bath.So while this proposed mechanism may or may not have some relevanceto some home experiments, it's irrelevant for most publishedresults. Supercooling Finally, supercooling may be important to theeffect. Supercooling occurs when water freezes not at 0Â°C, but atsome lower temperature. This happens because the statement that"water freezes at 0Â°C" is a statement about the lowest energy stateof the water -- at less than 0Â°C, the water molecules "want" to bearranged as an ice crystal. This means that they will stop zoomingaround randomly as a liquid, and instead form a solid ice lattice.However, they don't know how to form themselves as an ice lattice,but need some little irregularity or nucleation site to tell themhow to rearrange themselves. Sometimes, when water is cooled below0Â°C, the water will not see a nucleation site for some time, andthen water will cool below 0Â°C without freezing. This happens quiteoften. One experiment found that the initially hot water wouldsupercool only a little (say to about -2Â°C), while the initiallycold water would supercool more (to around -8Â°C) . If true,this could explain the Mpemba effect because the initially coldwater would need to "do more work" -- that is, get colder -- inorder to freeze. However, this also cannot be considered "the" sole explanation ofthe Mpemba effect. First of all, as far as I know, this result hasnot been independently confirmed. The experiment described above only had a limited number of trials, so the results foundcould have been a statistical fluke. Second, even if the results are true, they do not fully explain theMpemba effect, but replace one mystery with another. Why shouldinitially hot water supercool more than initially cold water? Afterall, once the water has cooled to the lower temperature, one wouldgenerally expect that the water would not "remember" whattemperature it used to be. One explanation is that the initiallyhot water has less dissolved gas than the initially cold water, andthat this affects its supercooling properties (see Dissolved Gassesfor more on this). The problem with this explanation is that onewould expect that since the hot water has less dissolved gas, andthus less nucleation sites, it would supercool more, not less.Another explanation is that when the initially hot water has cooleddown to 0Â°C (or less), its temperature distribution throughout thecontainer varies more than the initially cold water (see Convectionfor more on this). Since temperature shear induces freezing ,the initially hot water supercools less, and thus freezes sooner. Third, this explanation cannot work in all of the experiments,because many of the experiments chose to look not at the time toform a complete block of ice, but the time for some part of thewater to reach 0Â°C[7,10,13] (or perhaps the time for a thin layerof frost to form on the top ). While  says that it is onlya "true Mpemba effect" if the hot water freezes entirely first,other papers have defined the Mpemba effect differently. Since theprecise time of supercooling is inherently unpredictable (see ,e.g.), many experiments have chosen to measure not the time for thesample to actually become ice, but the time for which the sample'sequilibrium ground state is ice -- that is, the time when the topof the sample reached 0Â°C [7,10,13]. The supercooling argument doesnot 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 FrancisBacon" 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 ScientificRevolutions" 2nd edn (Chicago: University of Chicago Press), pg16 . 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 MedievalPhysics", AMS press, Inc., New York, 1967, pgs 72, 79, 94 Experiments on the Mpemba Effect . 5. Mpemba and Osborne, "Cool", Physics Education vol. 4, pgs172--5 (1969) . 6. Ahtee, "Investigation into the Freezing of Liquids", Phys.Educ. vol. 4, pgs 379--80 (1969) . 7. I. Firth, "Cooler?", Phys. Educ. vol. 6, pgs 32--41(1979) . 8. E. Deeson, "Cooler-lower down", Phys. Educ. vol. 6, pgs42--44 (1971) . 9. Osborne, "Mind on Ice", Phys. Educ. vol. 14, pgs 414--17(1979) . 10. M. Freeman, "Cooler Still", Phys. Educ. vol. 14, pgs417--21 (1979) . 11. G.S. Kell, "The Freezing of Hot and Cold Water", AmericanJournal of Physics, vol. 37, #5, pgs 564--5, (May 1969) . 12. D. Auerbach, "Supercooling and the Mpemba effect: When hotwater freezes quicker than cold", American Journal of Physics, vol.63, #10, pgs 882--5, (Oct 1995) . 13. J. Walker, "The Amateur Scientist", Scientific American,vol. 237, #3, pgs 246--7, (Sept. 1971) . 14. B. Wojciechowski, "Freezing of Aqueous Solutions ContainingGases", Cryst. Res. Technol., vol. 23, #7, pgs 843--8 (1988) General discussion on the Mpemba Effect . 15. New Scientist, vol. 42, #652, 5 June 1969, pg 515 . 16. New Scientist, 2 Dec. 1995, pg 22 . 17. New Scientist, vol. 42, #654, 19 June 1969, pgs 655--6 . 18. New Scientist, vol. 43, #657, 10 July 1969, pgs 88--9 . 19. New Scientist, vol. 43, #658, 17 July 1969, pgs 158--9 . 20. New Scientist, vol. 43, #658, 25 Sept. 1969, pg 662 . 21. New Scientist, vol. 44, #672, 23 Oct. 1969, pg 205 . 22. New Scientist, vol. 45, #684, 15 Jan. 1970, pgs 125--6 . 23. New Scientist, vol. 45, #686, 29 Jan. 1970, pgs 225--6 . 24. New Scientist, 2 Dec. 1995, pg 57 . 25. New Scientist, 16 Mar. 1996, pg 58 Related Articles . 26. J. Elsker, "The Freezing of Supercooled Water", Journal ofMolecular Structure, vol. 250, pgs 245--51 (1991) . 27. R.A. Brewster and B. Gebhart, "An experimental study ofnatural convection effects on downward freezing of pure water",Int. J. Heat Mass Trans. vol. 31, #2, pgs 331--48 (1988) . 28. R.S. Tankin and R. Farhadieh, "Effects of ThermalConvection currents on Formation of Ice", Int. J. Heat Mass Trans.,vol. 14, pgs 953--61 (1971)
Two small identical containers (ice cube trays, for example) withequal volumes of water are placed in a freezer - the onlydifference is one container holds hot water, and the …other oneholds cold water. The container that holds hot water freezes first.Why? . It is a misconception that hot water freezes sooner thancold water because of the rate of cooling alone. It is true that,at first, the hot water cools at a much more rapid rate than thecool water, due to the larger temperature differential of the hotwater container. But as the hot water container reaches the sametemperature as the cold water container, the temperaturedifferentials become the same, and from that point on, bothcontainers will cool at the same rate. But remember that the coldwater container was cooling too (just not as quickly), so it willalways have a head start, and the hot water should never be able tocatch up to the cold water before the cold water freezes. So theremust be something else going on than just cooling to enable the hotwater to freeze before the cold water. . The high temperaturedifferential with the hot water leads to something beyond justcooling. Hot water evaporates much more quickly in afreezer than cold water does. So by the time the hot water cools tothe same temperature as the cold water, there is much less water in the hot water container than water in the cold watercontainer. Now that they are the same temperature, they both shouldevaporate and cool at the same rate, so it is just a race to whichone freezes first. But with less water in its container, the(formerly) hot water has a higher surface area to volume ratio thanthe (originally) cool water. The higher surface area to volumeratio means the heat escapes more quickly (it cools more rapidly)from the (formerly) hot water. It is this higher surface area tovolume ratio, cased by the lower volume of water due toevaporation, that causes the (originally) hot water to freezefirst. . To prove this, conduct the following experiment. . . Place two identical ice trays in the freezer - one filled withhot water, and one filled with cold water. The hot water tray willfreeze first, but you will notice that there is much less ice leftin each ice cube in the hot water ice tray. . Now repeat the exact same experiment, but this time seal boththe hot water ice tray and the cold water ice tray with plasticcling wrap to prevent the water from evaporating. Now the coldwater ice tray will freeze before the hot water ice tray, and bothtrays will have equal amounts of ice. Probably not. It would freeze faster than hot water because the hotwater has to lose more heat.
Yes. Water of any starting temperature must be cooled to 0 o C in order to freeze.
Think about it this way... If you start out with hot water, it has to cool its freezing temperature before it will freeze. Somewhere along the process it will become the sa…me temperature as "cold" water - whatever temperature you choose to define as "cold". It takes time to reach that "cold" temperature and from the point that it becomes cold until it freezes should be the same as it was for water that started out "cold". All else being equal, cold water will freeze faster than hot water. Notice that I said "all else being equal". There are situations where water that starts out hot may freeze faster than the cold water. As an example - if you fill two ice trays with water - one with cold water and one with hot - and stick them into the freezer, the hot tray will start melting any frost or ice it is placed on. As it does so, it provides better contact with a cold surface that will act as a heat sink as it cools down. Having formed this better contact, it will begin to cool through both conductive and convective heat transfer. The tray that started out cold but which has poor thermal contact with its surroundings mainly cools by convection. Because conductive heat transfer is usually faster than convective heat transfer, the initially hot tray may catch up to the temperature of the initially cold tray and then continue to cool faster because of the boost from conductive heat transfer. Notice that the conditions have to be right for this to occur - the hot tray has to be warm enough to do the melting and establish the good surface-to-surface contact for the conductive heat transfer; there has to be frost or ice for it to be melting, the cold tray can't be too cold or it will start freezing before the hot tray catches up; likewise, the hot tray can't be too hot or it won't catch up to the cold tray; the freezer can't have too much forced convection (fan blowing in the freezer) but rather be primarily natural convection; the cold tray can't be warm enough to melt the ice or frost like the hot tray.
Actually hot water freezes faster then cold water this is known asthe Mpemba effect. The effect was first observed by Aristotle inthe 4th century BC.
Known as the Mpemba Effect after the one who bought it to modern notice. Yet too be explained satisfactorily, but much experimented. Archimedes knew that to have some wa…ter freeze soonest, it should be sun-warmed first. The optimum temperatures for your two glasses of water is 35 o C and 5 o C. Place each glass on an insulating mat in the freezer, and proceed.
Yes. Hot water freeze faster than cold see Mpemba Effect.
No, hot water does not freeze faster since hot water has molecules that move around faster than frozen water. It would take a higher cooling point before it would freeze.
answ2. This was first bought to the attention of science in recent times by Mpemba, and the Mpemba Effect is named after him. Optimal temperatures for this astonishing expe…riment are 35 o C for the hot liquid, and 5 o C for the cold one. Still not understood - maybe there is a Nobel in it? Archimedes noticed that in order to freeze a container of liquid, it was better to have it sun warmed first.
Yes, hot water freezes faster than cold water as seen in the Mpemba effect.
Because the particles in the cold water are slower than the particles in hot water and they become slower and slower until it freezes.
Warm water doesnt actually freeze faster. Distilled water (which you can get from boiling and collecting the steam) freezes at 0C or 32F. Water (such as tap water) with impuri…ties will only freeze at a lower temperature.
Hot water freezes faster than cold because of the Mpemba effect because water molecules move by a stream of moving cytoplasm which causes the hot water that its molecules to m…ove extremely fast using up a lot of kinetic energy so when in contact with a cold source all of the kinetic energy is sucked out of the water
No. Imagine if you have two people falling from the sky. One person is 1000 feet up while the other is 10. The one closer to the goal (which in your case is freezing water) wi…ll get there first. But just think about it. Why would you even want to freeze hot water?!
Sounds as if you were trying to repeat the Mpemba Effect . Apparently it was even known to Aristotle that to get a liquid to freeze faster, "warm it in the sun first". E…nter Mpemba (circa 1950s?) who had known that warm liquids freeze faster than cool ones when placed in the frig. Much experiments and $$ later, the explanation is yet to arrive (maybe a Nobel?), but the optimum temps for the experiment are 35 o C for the hot liquid and 5 o C for the cold one.
The question is unclear - Which, if not both, temperature waters are insulated? Do the insulations, if both, vary? I am assuming we are purely talking liquids freezing here …rather than including vapour. Cooling water down relies on the transmission of heat away from it so, assuming similar insulation: Other combinations - No, cold would freeze first. In similar conditions, hot water still has to loose the heat energy and become cold in addition to the time to chill from cold to frozen. Hot uninsulated vs Cold insulated - Possibly, depending on the insulation. If the energy loss were reduced for the cold water then if could be the case that hot water would freeze first. :-)