General > General Technical Chat
Freezing Speed of Hot Versus Cold Water
bdunham7:
--- Quote from: T3sl4co1l on February 23, 2022, 09:05:50 am ---So, if I understand this effect correctly, it would indeed make snow faster (nevermind the expense of heating all that water), because the transient heat capacity is lower, heat transfer is normal, and as a result it freezes faster. BUT, it would make a rather dense pack, or just not freeze very well at all, because the excess heat still has to come out at some point, sooner or later, and, I guess it's going to be mushy, or partially frozen or something until then?
How does that work anyway, does it actually freeze solid and the ice retains some anomalous heat? Even though it's a completely different phase?
--- End quote ---
Either the low heat capacity condition related to the O-H bond length would have to both survive the phase change and then relax slowly enough that the heat could be dissipated to the environment without any melting or there would have to be an alternative form of ice, like Ice-9. Or something I haven't thought of.
--- Quote ---So, I'm open to the phenomenon, and mechanism; but like I said, it's an extraordinary claim, and spanning 17 orders of magnitude with one mechanism requires a lot more motivation and explanation than I've seen here. For comparison, consider the decay of singlet oxygen: an extremely prohibited transition (due to quantum spin transition rules), so it persists for a relatively long time (seconds), typically being catalyzed by intermolecular interactions, or impurities, which break the symmetry and ultimately extract that energy, allowing decay to the ground (triplet) state. Such a remarkable and (nearly) "iron clad" quantum prohibition would seem necessary here -- but neither paper is presenting such a claim (and, I'm not aware of such states in bulk water -- again, not that I'm any kind of expert on that!).
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The main issue I have with the theory is with applying the claim broadly without any broader experimental evidence. I also have an issue with the conflation of skepticism with stupidity, but that's another matter.
A very simple possible explanation of Mpemba is available based on supercooling. If you supercool water, it may not always freeze quickly once it starts simply because the crystallization releases heat to the remaining liquid and additional heat rejection is required. If your freezer isn't very cold--say -10C--and you have a sample of water that will supercool to -5C before freezing starts, then the latent heat of fusion will be transferred slowly, proportional to the 5C difference. If you also have another sample of water that is crystallizing at 0C, then that system will be transferring the heat twice as fast based on the 10C difference. So all you need is a mechanism by which one sample is susceptible to supercooling and the other is not, the right ambient temperature and some luck. This would explain why the legend exists, but the phenomenon is not reliably observed experimentally, at least not in the ice-cube tray model. This is why I suggested that any experimental measurement should be continued until equilibrium is reached and certainly not stopped at 0C.
IanB:
--- Quote from: T3sl4co1l on February 23, 2022, 09:05:50 am ---So, I'm open to the phenomenon, and mechanism; but like I said, it's an extraordinary claim, and spanning 17 orders of magnitude with one mechanism requires a lot more motivation and explanation than I've seen here.
--- End quote ---
Actually, no. What it needs is an experimental procedure, clearly and precisely defined, so that any worker in any laboratory can reproduce the effect consistently.
So far, this does not appear to have been done. Where experimenters have claimed this effect, others have not been able to reproduce the same numbers.
Until there is a real, reproducible, observable phenomenon it is right to remain skeptical.
Computer simulations and theoretical models explaining how such a phenomenon might happen are interesting on an intellectual level, but they all remain hypothetical until they are able to give a precise specification for an experiment that will demonstrate in practice what the equations predict.
T3sl4co1l:
Actually, no -- it needs both! An experiment is fine, but teasing out conflating variables can be tricky. It's one thing to point at an experiment doing the thing and say "ha, see?", but conclusive proof requires both angles.
For example, point x-rays at it -- if it's bond lengths, then the diffraction pattern sure as hell ought to change. It'll change anyway due to temperature (I would think?), but there should be an aberration in the distribution if it really is energy stored in bond lengths.
Or also, would that change density somewhat? Enough to measure? It's effectively a metastable phase of liquid water, that must count for something?
Another example of experimental ambiguity is just saying "oh, it freezes faster". Well, what, it reaches 0C faster? Or it drops below 0C (fully solid phase) faster?
--- Quote from: bdunham7 on February 23, 2022, 02:56:42 pm ---A very simple possible explanation of Mpemba is available based on supercooling. If you supercool water, it may not always freeze quickly once it starts simply because the crystallization releases heat to the remaining liquid and additional heat rejection is required. If your freezer isn't very cold--say -10C--and you have a sample of water that will supercool to -5C before freezing starts, then the latent heat of fusion will be transferred slowly, proportional to the 5C difference. If you also have another sample of water that is crystallizing at 0C, then that system will be transferring the heat twice as fast based on the 10C difference. So all you need is a mechanism by which one sample is susceptible to supercooling and the other is not, the right ambient temperature and some luck. This would explain why the legend exists, but the phenomenon is not reliably observed experimentally, at least not in the ice-cube tray model. This is why I suggested that any experimental measurement should be continued until equilibrium is reached and certainly not stopped at 0C.
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...As exemplified here.
Which is something else XRD would show, phase fractions. With the ice phase giving sharply defined rings or whatever.
If the toss-a-pot-of-hot-water-on-a-cold-day trick is indeed an example of this effect, then it would seem the former is relevant -- given that the "control" doesn't freeze much or at all!
Of course you have to be careful that the ionization doesn't also cause nucleation, where supercooling might be involved. But it would be nice to show it's not a supercooling phenomenon but something different entirely.
Sort of related: https://arxiv.org/ftp/arxiv/papers/1107/1107.4795.pdf
And, any other low-impact analyses would be effective, too; maybe Raman something or other, would be more "hands free" than ionizing radiation? Or, there's a zillion other probing methods I don't even know about, probably some neat things to try among them...
Tim
IanB:
--- Quote from: T3sl4co1l on February 23, 2022, 06:00:17 pm ---Actually, no -- it needs both! An experiment is fine, but teasing out conflating variables can be tricky. It's one thing to point at an experiment doing the thing and say "ha, see?", but conclusive proof requires both angles.
--- End quote ---
Why would you say conclusive proof requires an explanation of a mechanism? For much of history, people knew that certain things happened, but they didn't know why. For instance, everyone knew for millennia that if you seared meat it got a brown color with a pleasing flavor. The mechanism for why it happened was not known, but the observation was nevertheless an incontrovertible fact. Only later was it discovered that it is due to a chemical reaction between amino acids and reducing sugars, and today we can call it the Maillard reaction.
Turn it around, and consider general relativity. Einstein made predictions based on a mathematical model, but those predictions were only a hypothesis until confirmed by observation. Without the real world observations, general relativity would remain an intellectual curiosity, rather than a testable theory.
The Mpemba effect can only happen if water or another substance exhibits thermal hysteresis. You can come up with a theoretical mechanism to produce such hysteresis, but then it should be possible to construct an experiment with the right elements to observe the effect in reality (as can be done with magnetic hysteresis). Without such experiments the effect is only "real" in the mathematical, theoretical sense. It is not real in the observational, experimental sense.
T3sl4co1l:
Yes, exactly -- I mean as a well proven, understood, scientific theory. As opposed to a phenomenon (observation lacking explanation), or hypothesis (explanation lacking observation).
Tim
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