# Science of Freezing of Hot Vs Cold Water

Dr. S. S. VERMA; Department of Physics, S.L.I.E.T., Longowal; Distt.-Sangrur (Punjab)-148 106

2020-09-06 14:35:36

Credit: pixabay.com

In the late of17th century British scientist Isaac Newtongave a well-known and important law explaining the cooling of a body by radiating its heat to its surroundings. According to the law, rate of loss of heat (dQ/dt) or cooling of body is directly proportional to the temperature difference between the body and its surroundings, given as: dQ/dt α (TS−T) =>dQ/dt= βA(TS−T), whereQ is the heat, Ais the surface area of the body through which the heat is transferred, T and TSis the temperature of the body and surrounding environment respectively, whereas β is the heat transfer coefficient and depends on thegeometry of the body, state of the surface, heat transfer mode, and other factors.As Q=CT, where Cis theheat capacityof the body, it can be written as dT/dt=αAC(TS−T)=k(TS−T) and the given differential equation has the solution in the form:T(t)=TS+(T0−TS)exp (−kt), where T0denotes the initial temperature of the body.The cooling rate depends on the parameterk= βAC. With increase of the parameter k(for example, due to increasing the surface area), the cooling occurs faster (see Fig. for k1 and k2values.

If other conditions remain same, it is clear from the law that cooling of the body directly depends on the temperature difference between the body and its surroundings. From this, it is generally said that bodies of same nature at higher temperature cool faster as compared to cold ones in the same environment. However, as the loss of heat is exponential in nature and therefore it is faster for hot body initially but at longer times the rate of loss almost coincides for both hot and cold bodies and the rate of loss almost becomes same. Now, the question which is always asked is weather a hot water will freeze faster than cold in a freezer. It is to mention that hot water can in factfreeze faster than cold waterfor a wide range of experimental conditions and this phenomenon is extremely counterintuitive, and surprising even to most scientists. But this is in fact a real phenomenon and has been seen and studied in numerous experiments. In this article, we will try to explore the physics behind the process and try to bring out the answer to the question.

The Mpemba effect is a process in which hot water can freeze faster than cold water and the phenomenon is temperature-dependent. It is named after Tanzanian schoolboy Erasto Bartholomeo Mpemba (born 1950) who discovered it in 1963 at the age of thirteen years only. Ashot waterplaced in an open container begins to cool, the overall mass decreases as some of thewaterevaporates and thus with less watertofreeze, the process can take less time, thus, evaporation is the strongest candidate to explain the Mpemba effect.There is still some disagreement about the parameters required to produce the effect and about its theoretical basisbut in 2020Naturepublished a paper that sought to "outline the generic conditions needed to accelerate heat removal and relaxation to thermal equilibrium and support the idea that the Mpemba effect is not simply a scientific curiosity concerning how water freezes into ice - one of the many anomalous features of water - but rather the prototype for a wide range of anomalous relaxation phenomena of broad technological importance".

The experiments studying this phenomenon have always been muddled by the complexities of water and the freezing process, making results difficult to reproduce and leaving scientists disagreeing over the cause of the effect, its definition and whether it is real or not.Recently, a new experiment with the study of tiny glass beads suggests that the Mpemba effect is real andhints at how hot water can freeze faster than cold.Any warmer systemwhen chilled to cool off takes less timethan a cooler system to reach the same low temperature. And in some cases, the speedup was even exponential, physicists report in the Aug. 6, 2020Naturemagazine.To sidestep the experimental complexities, Avinash Kumar and John Bechhoefer, both of Simon Fraser University in Burnaby, Canada, used tiny glass beads, 1.5 micrometers in diameter, in lieu of water and tried to define Mpemba effect based on cooling instead of the more complicated process of freezing. According to the researchers, this is the first time that an experiment can be claimed as a clean, perfectly controlled experiment that demonstrates this effect.