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Freezing Speed of Hot Versus Cold Water
PlainName:
--- Quote ---The point is simply that you need to observe that long enough and far enough down that you can be sure that the phase change is complete. Then you can go back and look at the data to see when you deem the sample 'frozen'.
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But in doing that you are merely picking some semi-random point at which you say "should be done now". The samples could have been completely frozen much earlier, or even later, and you won't know exactly when, but your confirmation bias will ensure the result you seek is apparent.
And in the end you won't be able to say why the repeatable effect shouldn't happen, only that it hasn't for your crude and unrepeatable experiment. OTOH, NA has quoted sources that, if the effect is real, can explain precisely how it can be real.
bdunham7:
--- Quote from: dunkemhigh on February 25, 2022, 12:58:49 pm ---But in doing that you are merely picking some semi-random point at which you say "should be done now". The samples could have been completely frozen much earlier, or even later, and you won't know exactly when, but your confirmation bias will ensure the result you seek is apparent.
And in the end you won't be able to say why the repeatable effect shouldn't happen, only that it hasn't for your crude and unrepeatable experiment. OTOH, NA has quoted sources that, if the effect is real, can explain precisely how it can be real.
--- End quote ---
What do you mean by 'semi-random point'? I think that most would agree that the completion of the phase change would be indicated by the temperature starting to fall again after a plateau. How would you determine at what point the samples are indeed 'frozen'?
PlainName:
--- Quote ---What do you mean by 'semi-random point'?
--- End quote ---
It's not completely random since it's limited to domestic refrigerator capabilities. Water freezes at 0C, so why not pick that? OTOH, if there are impurities then it will be a bit different, so how are you going to cope with unknown quantities of those?
Further, the lower ambient temperature would reduce the time available to observe the effects (and increase the likelihood of partial freezing). Shove the ice cube tray in liquid nitrogen and you'd be hard pressed to tall the exact time to the ms when it all froze. Use ambient of -5C and you have much much longer to determine things (and you won't have a crusty shell and fluid interior).
So, why -20C. No real reason other than that's what your freezer is set to. It's a semi-random figure.
bdunham7:
--- Quote from: dunkemhigh on February 25, 2022, 04:25:06 pm ---Water freezes at 0C, so why not pick that?
--- End quote ---
Do you mean use an ambient (freezer) temp of 0C or terminate the experiment at 0C? Either way, how can you determine that freezing has occurred?
--- Quote ---So, why -20C. No real reason other than that's what your freezer is set to. It's a semi-random figure.
--- End quote ---
I haven't proposed that -20C is the correct or only temperature to use. In fact I proposed a possible mechanism by which the Mpemba effect might be observed some times and not others and also that experiments with varying freezer temps might reveal that. Did you read that? However, -20C, in addition to being approximately what my freezer is set to, is also cold enough that supercooling is unlikely and you can be fairly sure that the water is completely frozen if you allow it to come near equilibrium. So if I were going to conduct a set of experiments, in the first round at least I would use -20C as my coldest test and then successively warmer temps up to near 0C. Unfortunately, both my freezers have a lot of food in them so I won't be doing this anytime soon.
--- Quote ---Shove the ice cube tray in liquid nitrogen and you'd be hard pressed to tall the exact time to the ms when it all froze.
--- End quote ---
In that case it might actually not turn into conventional ice. Ice-9 is fiction, but there is another amorphous state that occurs when you cool water so fast the the ice crystals don't have time to form before things stop moving.
PlainName:
--- Quote ---Do you mean use an ambient (freezer) temp of 0C or terminate the experiment at 0C?
--- End quote ---
Terminate.
--- Quote --- Either way, how can you determine that freezing has occurred?
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Well, presactly. That's the problem, isn't it? And you not only have to determine that freezing has occurred, but when it occurred.
--- Quote ---I haven't proposed that -20C is the correct or only temperature to use.
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It was implied when you said: "but the ultimate question for me is whether I should use hot or cold water in my ice cube tray."
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