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| Freezing Speed of Hot Versus Cold Water |
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| SL4P:
I remember this experiment from year 8 in school. The significant factor is the ‘rate of change’, not the immediate temperature of the samples. |
| tunk:
Were they both fully frozen when you stopped the test? If they were, I would have expected a further drop down to ambient. And it may have been better to put the sensors centrally in the tin. |
| Rick Law:
The Carl Sagan "standardard" : "Extraordinary Claims Require Extraordinary Evidence." This "Mpemba effect" is an extra-ordinary claim. It is trying to invalidate the "Intermediate Value Theorem" long established in mathematics. For "Mpemba effect" to be true, the hotter one must get pass the temperature of the cooler one without ever taking on the temperature value of the cooler one. I believe the "Mpemba effect" is a description of poor experimental techniques. Here is why: Mpemba's observation was in 1963 by: "beakers in the icebox of a domestic refrigerator on a sheet of polystyrene foam". A 1960 style refrigerator likely is fan-less inside and relies solely on convection to "spread the cold" within. The evaporator may be directly connected to the ice box or is the ice box itself. There is no way to ensure that the two beakers are under the exact same condition. Even with modern refrigerators with fan-forced air circulation, you cannot ensure the conditions for the exact same physical spot will have the exact same condition over time. "Beakers in the icebox" implies they were done together. When you placed the two beakers in together, they cannot both be at the same spot. Two different spots will have two different conditions. Where is the inlet of the refrigerant (coolest)? What else may be inside the refrigerator affecting circulation(s)? Etc., etc Let say I infer wrong. May be the two beakers were at the same spot and test one after the other. Again, conditions will be different for the two of them because they were there at different time. Immediately, one can reason that with something hotter inside the refrigerator, the average temperature inside will be higher and the cooling motor must run more to force to cool the refrigerator's thermostat enough to shut down the motor. So the hotter beaker was cooled more than the other. For such " observation " to be useful, One must document in detail how the conditions for these beakers are controlled and how exact same-ness was attained. Otherwise, one may just be observing the effect that cooling ability inside a refrigerator varies a lot, by time and by location. If one place a thermometer inside a fridge, one would be amazed at how much the temperature really varies by time and placement. I did, right after I replaced the defunct thermostats in my fridge to validate it works. I still have the two digital thermostat connected: one sensor inside the freezer compartment and one inside the fridge compartment wired to displays outside. So I have been observing the freezer/fridge temperature for 2-3 months now. Furthermore, even what "freezing" means is not even defined in reporting this "observation". This lack of definition is very sloppy! Does it mean reaching freezing temperature? Water at freezing temperature still need to loose more latent heat for phase change to complete there by turned solid. The best way to check for phase change is when the temperature momentarily stops changing. That "temporally constant temperature" tells the observer that phase change is occurring. Is that what he means by freezing? How is the phase change observed? Hence my opinion: Poor documentation, poor understanding of physics experimental techniques, and poor definition. Very sloppy indeed. Good initiative and good curiosity however. |
| Nominal Animal:
--- Quote from: Rick Law on February 18, 2022, 10:28:48 pm ---For "Mpemba effect" to be true, the hotter one must get pass the temperature of the cooler one without ever taking on the temperature value of the cooler one. --- End quote --- No; must pass the temperature of the cooler one while having smaller thermal capacity than the originally cooler one. One of the experimental verification methods is to choose a temperature, say +27°C = 300 K. Put say 200g of pure water in a stoppered beaker in a temperature bath for say a half hour. In a column, boil pure water and have it run through a powerful cooling sleeve (a pipe inside a pipe, with the coolant running in the outer pipe), at such a flow rate that the output is very close to the target temperature, and measure the exact same amount into an identical beaker. At this point, you should have two beakers with the same amount of pure water at the same temperature; the only difference being that one has been at that temperature for some time, the other just flash-cooled from near boiling. Then, put the two into the same liquid nitrogen bath (-196°C = 77 K), and measure the temperature changes in the two. The current understanding is that indeed, one will cool faster than the other; and this is what the "Mpemba effect" actually refers to. For reasons why, see my earlier post. If you need clarifications, I'd be happy to elaborate; it is interesting but complicated, surprising but not that weird at the atomic level. It's like the fact that solid water is less dense than liquid water at +4°C = 278 K. |
| rfeecs:
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