?Why Hot Water Cools Faster Than Cold Water (January 2018)Mahmoud Shazly, Master Student, Coburg University of applied sciencesAbstract- We observed the Mpemba effect, initially hot water freezes faster than initially cold water. “Although the effect might appear impossible, it has been observed in numerous experiments and was discussed by Aristotle, Francis Bacon, Roger Bacon, and Descartes. It has a rich and fascinating history, including the story of the secondary school student, Erasto Mpemba, who reintroduced the effect to the twentieth-century scientific community”. The phenomenon is simple to describe and illustrates numerous important issues about the freezing of hot water faster than cold water. “The role of skepticism in scientific inquiry, the influence of theory on experiment and observation, the need for precision in the statement of a scientific hypothesis, and the nature of falsifiability. Proposed theoretical mechanisms for the Mpemba effect and the results of contemporary experiments on the phenomenon are surveyed”. We also observed different factors on which mpemba effect depends. 4Index Terms— Thermal contact, Evaporation, Degassing, Supercooling, Distribution of solutes, Impurities, and depth of supercooling of cold water.I. INTRODUCTION “My name is Erasto B Mpemba, and I am going to tell you about my discovery, which was due to misusing a refrigerator.””With those words, Tanzanian student Erasto Mpemba entered scientific history, and also sparked a scientific mystery and controversy that remains ongoing today, some 40 years later!The phenomenon Mpemba found is now known as the Mpemba effect, and is the very counterintuitive idea that, under certain circumstances, a quantity of very hot/boiling liquid can freeze faster than an equal quantity of cold liquid”Mpempa used to make ice cream while he was in secondary school and one day he observed that hot liquids freeze faster than cold liquids.” 1This article will try to show the closer explanation for this mysterious phenomena. II. THEORY EXPLANATIONWe observed that two identical beakers of water, placed at two different temperatures, put in contact with a thermal reservoir at subzero (on the Celsius scale) temperature. “While one may intuitively expect that the initially cooler sample would freeze first, it has been observed that this is not always the case. This paradoxical behavior named the Mpemba effect (ME) has been known since antiquity and discussed by philosophers like Aristotle, Roger Bacon, Francis Bacon, and Descartes. Nevertheless, physicists only started to analyze it in the second part of the past century, mainly in popular science or education journals”.There are no clearly defined physical mechanisms that explain the Mpemba Effect. “Specifically, water evaporation, differences in the gas composition of water, natural convection, or the influence of supercooling, either alone, or combined with other causes, have been claimed to have an impact on the ME. Conversely, the own existence of the ME in water has been recently put in question. Notwithstanding, Mpemba-like effects have also been observed in different physical systems, such as carbon nanotube resonators or clathrate hydrates”.The ME needs the evolution equation for the temperature to include other variables, “which may facilitate or hinder the temperature relaxation rate. The initial values of those additional variables depend on the way the system has been prepared, i.e., “aged,” before starting the relaxation process. Typically, aging and memory effects are associated with slowly evolving systems with a complex energy landscape, such as glassy or dense granular systems. However, these effects have also been observed in simpler systems, like granular gases or, very recently, crumpled thin sheets and elastic foams .” 2II. REASONS1. Better thermal contact”Better thermal contact can result in hot water losing heat faster and therefore freezing before cold water under certain conditions.4 The hot water containers will often make better thermal contact with the cooling environment because the hot container melts more freezer frost than the cold container. When the melted freezer frost refreezes under the containers the results are often a faster flow of heat from the container that is in better thermal contact with the freezer, this will usually be the hot container. The so-called “frost free freezers” are not truly frost free all of the time” 3.2. EVAPORATIONA boiling or very hot liquid losses some of its mass due to evaporation because of phase transition from the liquid into gas. With a lower mass, “it will cool faster, possibly giving a “boost” to the Mpemba effect. Osborne already noted that evaporation alone could not account for all of the rates of cooling of the hot liquid.” 13. DEGASSINGSeveral experiments had been performed in 1988 and a Polish research group reasons reintroduced the Mpemba effect and observed that the effect strongly depends on the amount of gas dissolved in the water. “When the water was purged of air and carbon dioxide, the time to freeze became proportional to the starting temperature. The researchers suggested that the presence of gas was substantially slowing the rate of cooling and that the heated water was purged of it.” 14. SUPERCOOLINGIn 1995, the German scientist, David Auerbach, claimed that the Mpemba effect can be explained by using the factors based on supercooling, and performed experiments to back up the assertion. At freezing point, the liquid starts freezing with the help of impurities around which the crystals can nucleate. “In the absence of such impurities, the liquid can be cooled below its normal freezing point while remaining a liquid — it is “supercooled”. Auerbach suggests that the cold water will super cool to a lower temperature than the hot water, thus giving the hot water an “edge”. However, the reason that why hot water has a higher supercooling temperature is still unclear, and possibly related to the earlier noted effects.” 15. DISTRIBUTION OF SOLUTESIn 2009, again an interesting explanation was introduced by J.I. Katz, he suggested that in the presence of solutes cold water freezes slowly, as suggested earlier, “but also that those solutes get driven from the freezing water into the as-yet unfrozen water, slowing the process further.” 16. IMPURITIES”Impurities are found in all bulk water and bulk water containers, no matter how pure or clean the water is”. That is why bulk water cannot be cooled to the heterogeneous nucleation temperature of approximately -40oC. The agents are usually in the water while the sites may be affixed to the inside of the containers. These sites or agents all have different temperatures at which they can interact with the molecules of water causing a phase transition from liquid to crystalline like structure that becomes a seed for ice nucleation and freezing. “The site or agent with the highest “ice nucleation temperature” determines at what temperature a sample of Still water in a closed container will begin freezing. Notice how reproducible the ice nucleation temperature was a new “ice nucleation site” was produced at a fracture site in the glass vial, Fig. 10. Notice also that this new site has a higher “ice nucleation temperature” and that the water begins the freezing process at this higher temperature. Figure 8 on page 34 in the same paper shows an example of ice nucleation occurring near the top of the water in a vial.” 37. DEPTH OF SUPER COOLING OF COLD WATERWhen hot water starts freezing faster than cold water, keeping all conditions identical except initial temperature, the cold water always supercools to a much lower temperature than the hot water. “Hot water will consistently freeze before cold water only when the cold water supercools to a much lower temperature than the hot water, at least ~5.5oC.” 3III. CONCLUSIONIt has been observed that hot water freezes faster than cold water is usually a chance occurrence which until now could not be repeated on demand. It happens sometimes in some certain conditions. That is why the mystery has survived for more than 2000 years. I was able to succeed in observing hot water freeze faster than cold water 28 times, consecutively. “This can only be observed when the water samples are in closed Containers and pre-selected as discussed above. If your object is to freeze a container of water quickly, then placing an open container of hot water on a bed of freezer frost is a very good idea. The hot water will most likely cool to 0oC before a container of cooler water because of better thermal contact with the cold surface. Microscopic ice crystals (freezer frost) forming in the air and on the containers, will be the seed crystals that start the freezing process in the first water sample that cools to 0oC”. However, the cold water will freeze first if both containers are placed on thermal insulators because it will always cool to 0oC first. “Put a lid on the containers and it will be seeds already in the water and or containers that will determine which freeze first not necessarily the first one to cool to 0oC”. 3References1 Mpemba’s baffling discovery: can hot water freeze before cold? (1969) | Skulls in the Stars on WordPress.com. Online Available: https://skullsinthestars.com/2011/05/31/mpembas-baffling-discovery-can-hot-water-freeze-before-cold-1969/. Accessed Jan. 16 2018.2 A. Lasanta, F. Vega Reyes, A. Prados, and A. Santos, “When the Hotter Cools More Quickly: Mpemba Effect in Granular Fluids,” (eng), Physical review letters, vol. 119, no. 14, p. 148001, 2017.3 H. C. Burridge and P. F. Linden, “Questioning the Mpemba effect: Hot water does not cool more quickly than cold,” Scientific Reports, vol. 6, p. 37665, https://www.nature.com/articles/srep37665.pdf.4 M. Jeng, “The Mpemba effect: When can hot water freeze faster than cold?,” American Journal of Physics, vol. 74, no. 6, pp. 514–522, 2006.