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What is your explanation for the Mpemba effect?

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posted on Feb, 9 2017 @ 02:35 PM
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For over 2000 years, since the time of Aristotle, people have been reporting that hot water freezes faster than cold water. This effect is now named after the Tanzanian student Erasto Mpemba who in 1963 was making ice cream in cooking classes and found that putting a hot mixture in the freezer resulted in freezing faster than the cold mixtures that were in the same freezer. The common reports throughout history applied to this phenomenon occurring with water instead of ice cream.

About 7 years ago I read the available literature and I wasn't really satisfied with any of the explanations so I started doing my own crude experiments in my own freezer, and I wasn't finding the hot water to freeze any faster than the cold water. What I did find is that water froze faster on the top shelf than on the bottom shelf of the freezer, which sounds counter-intuitive since we are taught that warmer air rises and cooler air sinks which all else being equal is usually true, but all else was not equal in my freezer. There is a vent located behind the top shelf that was blowing on the water containers causing the containers located there to freeze faster, so I tried to isolate the containers from this source of error and other sources of error, and then I started wondering if all the other experimenters doing their experiments had identified all the possible sources of error. For example I don't know if the freezer Erasto Mpemba used had such a vent but if it did and if he placed his hotter mixture next to the vent, I can see in my own experiments how the hot mixture could freeze more rapidly yet be fully consistent with thermodynamic laws.

From the literature I read it was apparent that many had tried to identify and eliminate potential sources of measurement and experimental error, but I am not sure they succeeded.

When I saw that Derek from the youtube Veritasium channel had started a science channel and the first video was about the mpemba effect, I had to watch it. He covers 5 possible explanations that have been considered, and you can watch his video here:

Does Hot Water Freeze Faster Than Cold Water?

The Wikipedia article mentions 7 possible contributing factors.

So which of those explanations does Derek think causes the mpemba effect?

He cites a paper from 2016 which re-defined the effect to reaching a temperature of zero degrees C instead of freezing, but to me this is not what mpemba originally reported so I don't know if we can consider the mystery solved, but the researchers found that this experiment is particularly susceptible to small sources of measurement error. The authors describe that they broke the mpemba effect into a two-part process and they focused on the first part:

Questioning the Mpemba effect: hot water does not cool more quickly than cold

A reasonable start to analysing the problem is to consider the process in two stages; first, cooling the water to an average temperature of 0 °C (or enthalpy equivalent thereof), and second, freezing the water to form solid ice. In so doing any effects associated with the supercooling of water are entirely contained within the second stage. We restrict our definition of the Mpemba effect to the first stage of the process, i.e. the process of cooling a sample of warm water to 0 °C in less time than it takes to cool a sample of water, which is notionally identical except that it is initially at a lower temperature, to 0 °C.


So without a careful examination of the second part which could relate to supercooling I'm not sure this solves the mystery, but supercooling could play a role and help explain the inconsistent experimental results.

Derek also mentions that the "supercooling" hypothesis might explain why experimental results might be so inconsistent and difficult to reproduce since something as trivial as the presence or absence of a piece of dust might provide the nucleating agent to get the freezing started...or not if it's not there, resulting in supercooling.

So while I find the video informative, and feel somewhat vindicated that my crude experiments never replicated the mpemba effect but showed sources of experimental error, I'm not 100% sold on the idea that all experimental results can be explained this way so I'm still not sure if we have the answers to this 2000 year old question: why does hot water freeze faster than cold water, or does it?

I'd be interested to know if you think the reports of this happening for the last 2000+ years are real, or some kind of experimental error, and if you think you might know the cause if it's real. Have you ever observed anything like this yourself where hot water (or ice cream) freezes faster than cold?

edit on 201729 by Arbitrageur because: Added question mark to title




posted on Feb, 9 2017 @ 02:43 PM
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Alls i can contribute is this.
I dont know where i heard this, was a long time ago.

Ive always believed that hot water is "soft" and cold water is "hard"
I cannot expand further, thats all i know...



posted on Feb, 9 2017 @ 02:44 PM
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a reply to: Arbitrageur

Weird, I've never heard about this claimed effect before, thanks for sharing. S+F


I didn't try it yet. But something comes up to mind about the hot ice cream: fat. When you heat fatty acids, you saturate it. Saturated fats are known to solidify at higher temperatures than unsaturated fats. I wonder if cooking the thing increases its saturated fat ratio, and thus makes it solidify quicker once put in the freezer.



edit on 9-2-2017 by swanne because: (no reason given)



posted on Feb, 9 2017 @ 02:48 PM
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a reply to: SecretKnowledge

(put hot water in the freezer and cold too) I have and cold freezes faster.
edit on 9-2-2017 by seasonal because: (no reason given)



posted on Feb, 9 2017 @ 02:48 PM
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a reply to: Arbitrageur




The total entropy of an isolated system always increases over time, or remains constant in ideal cases where the system is in a steady state or undergoing a reversible process.


I thought the "hot water freezes faster than cold water" experiment was simply due to the 2nd law of thermodynamics, entropy

However, as your OP also suggests, there doesn't seem to be any "fully satisfactory theoretical proof" for the 2nd law of thermodynamics




There is no fully satisfactory theoretical proof for the Second Law, although there are some connections to Quantum Mechanics, Probability and Relativity.


Source

I'm not a physics nut so I could be completely wrong



edit on 9-2-2017 by FamCore because: (no reason given)



posted on Feb, 9 2017 @ 02:50 PM
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Can we have a definition of "freeze?"

(yeah. I wiki'ed it.)
edit on 2/9/2017 by Phage because: (no reason given)



posted on Feb, 9 2017 @ 02:53 PM
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a reply to: Phage

I second that question. I'm guessing "reach a solid phase".



posted on Feb, 9 2017 @ 02:53 PM
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a reply to: swanne
The entire sample?



posted on Feb, 9 2017 @ 02:57 PM
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a reply to: Phage

Dunno. Probably? however, If the experiment is about "rate of freezing", then the volume of the samples needing freezing is irrelevant, as long as the volumes of the samples which need freezing are equal with one another.


edit on 9-2-2017 by swanne because: (no reason given)



posted on Feb, 9 2017 @ 03:01 PM
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a reply to: swanne

Really? So, just a bit of surface ice could be the criterion?

Arb? What did you use for "freeze" in your experiments?



posted on Feb, 9 2017 @ 03:04 PM
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Hm.
It seems backward.

Hot molecules (less viscous) move about quicker than cold molecules (more viscous) do? The less movement the quicker the freeze?

Maybe trying motor oil in the test using a rated viscosity would eliminate errors?

Having a giant million square foot freezer and placing the two samples on the floor would produce a more accurate result?

Still seems backward to me.





posted on Feb, 9 2017 @ 03:05 PM
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a reply to: Phage

He used water. Which has no fat. And he reported that the experiment failed to display the effect. This is why I think saturation of fat is the possible culprit in the ice cream thing.



posted on Feb, 9 2017 @ 03:10 PM
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originally posted by: Phage
a reply to: swanne

Really? So, just a bit of surface ice could be the criterion?

Arb? What did you use for "freeze" in your experiments?
As the video explains the lack of a clear definition of the mpemba effect is part of the problem, but my definition was the same as the citation in the OP from the 2016 paper which is "freezing the water to form solid ice" so just a bit of surface ice wouldn't count, though in my experiments that distinction wouldn't have made any difference because the surface ice always appeared on the cooler water first unless I had sources of experimental error like the vent blowing on the top shelf.

If the effect occurs because of supercooling, we wouldn't expect the distinction of surface ice to matter, because once ice forms anywhere in supercooled water, it tends to spread everywhere. You're probably familiar with this but for anybody who isn't, here's a demo where you can watch how fast the ice spreads once it forms anywhere in supercooled water:

Supercooled Water - Explained!



posted on Feb, 9 2017 @ 03:16 PM
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originally posted by: swanne
a reply to: Phage

He used water. Which has no fat. And he reported that the experiment failed to display the effect. This is why I think saturation of fat is the possible culprit in the ice cream thing.
Yes my experiment failed to display it but some experimenters observed it with water, this researcher 28 times in 28 trials, and he thinks it's related to supercooling:

A search for the Mpemba effect: When hot water freezes faster then cold water

An explanation for why hot water will sometime freeze more rapidly than cold water is offered. Two specimens of water from the same source will often have different spontaneous freezing temperatures; that is, the temperature at which freezing begins. When both specimens supercool and the spontaneous freezing temperature of the hot water is higher than that of the cold water, then the hot water will usually freeze first, if all other conditions are equal and remain so during cooling. The probability that the hot water will freeze first if it has the higher spontaneous freezing temperature will be larger for a larger difference in spontaneous freezing temperature. Heating the water may lower, raise or not change the spontaneous freezing temperature. The keys to observing hot water freezing before cold water are supercooling the water and having a significant difference in the spontaneous freezing temperature of the two water specimens. We observed hot water freezing before cold water 28 times in 28 attempts under the conditions described here.


edit on 201729 by Arbitrageur because: clarification



posted on Feb, 9 2017 @ 03:17 PM
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a reply to: Arbitrageur

I first knew of the supercooling effect when I opened a Coke bottle that had been in the freezer, when I was 8 or 9. I had no idea what was going on but it was awesome.


That quote isn't exactly an explanation, btw. It just sort of kicks the can (Coke can?) down the road.

edit on 2/9/2017 by Phage because: (no reason given)



posted on Feb, 9 2017 @ 03:36 PM
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originally posted by: loveguy
Hm.
It seems backward.

Hot molecules (less viscous) move about quicker than cold molecules (more viscous) do? The less movement the quicker the freeze?...
Still seems backward to me.
According to just the laws of thermodynamics, it is backwards, because they say the cold water should freeze first. Some researchers think there is more going on here such as supercooling, and if that's the case, then the experiments don't violate the laws of thermodynamics as some might suggest.
edit on 201729 by Arbitrageur because: clarification



posted on Feb, 9 2017 @ 03:46 PM
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You can't put both hot and cold samples in a small freezer together at the same time as the warmth from the hot sample will change the conditions for the cold sample.



posted on Feb, 9 2017 @ 03:49 PM
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a reply to: Arbitrageur

Ooh, I've got an idea. Maybe the evaporation of hot water solidifies upon contact with the cold of the freezer, as meanwhile the main body of liquid reaches a supercooled state. When the frozen crystals from evaporation fall back on the supercooled fluid, they trigger an avalanche and the supercooled turns to ice.

Whereas cold water in the freezer won't evaporate much, and there'll be nothing to trigger the shift from supercooled to solid (though sheer cold will eventually do it).

Hence, hot water would freeze faster because of ice precipitation from its own evaporation.

Just a crazy idea.



posted on Feb, 9 2017 @ 03:57 PM
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a reply to: swanne

Stranger things have occurred with Helium 3 but this might be off topic.



posted on Feb, 9 2017 @ 03:57 PM
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originally posted by: TheAlleghenyGentleman
You can't put both hot and cold samples in a small freezer together at the same time as the warmth from the hot sample will change the conditions for the cold sample.
There are ways to mitigate the transfer of heat but if heat transfer is all that is taking place there should be no conditions for which the hot water will freeze first, because it doesn't have any way to transfer enough heat to the colder water to make it hotter than itself, right?



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