B How can boiling water freeze faster than water at room temp?

russ_watters

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I've said this in a previous thread, explain why. It does no help to just state something without providing an explanation to the statement. By explaining your statement we can get to the answer of this thread quicker and more efficiently.
Newton's law of cooling is based on the observed fact that heat flow is proportional to temperature difference. So if you start off with twice the temperature difference you get twice the heat flow. When the temperature of the "hot" water reaches the starting temperature of the "cold" water, it must continue following that law and follow exactly the same temperature profile that the "cold"water followed.
 

russ_watters

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my point is that thermal exchanges and newton's law of cooling works but if a paradox emerges ,the catch must be somewhere else!
Indeed that's true: but my point is that I've never seen convincing evidence that the paradox actually exists. All of these papers build mathematical models to attempt to explain something that hasn't been shown to be real and violates established science. That's putting the cart miles before the horse!
 
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Indeed that's true: but my point is that I've never seen convincing evidence that the paradox actually exists. All of these papers build mathematical models to attempt to explain something that hasn't been shown to be real and violates established science. That's putting the cart miles before the horse!
I agree, there is indeed no paradox, but the Mpemba effect is real nonetheless. Have you read the Brownridge paper referred to above? I think it addresses most issues we are discussing. He performed detailed measurements (in an actual lab) and he could create setups where the Mpemba effect never occurs and setups where it always occurs.

One issue is that 'everything else equal' (which is by the way not a requirement in the description of the Mpemba effect) means that the initial conditions of the experiment are exactly the same, except for the temperature. Of course, by the time the hot water has reached the initial temperature of the cold water, there must be a difference, in the boundary conditions or elsewhere, between the hot water setup and the cold water setup.
Another issue is that the point of freezing is usually not 0 degrees Celcius. When T1 < T2 and the boundary conditions stay the same, then the water at T1 will always cool to 0 C in a shorter time than the water at T2. But water will not always freeze at 0 Celcius, and supercooling is actually quite common. This supercooling is affected by nucleation sites present in the water and boiling the water affects the number of nucleation sites.

Issue 1: Brownridge presents a setup where you put the water in a copper container and then place it on the frost layer in the freezer. The cup of hot water will immediately melt the frost below the cup, which will then refreeze as ice. Of course, by the time that the hot water reaches the initial temperature of the cold water, the boundary conditions are completely different (thermal conductivity of the ice layer below the container is higher than the thermal conductivity of the frost layer).

Issue 2: Brownridge spends a lot of time on the supercooling effect and his conclusion is:
"The Mpemba effect, describing the phenomenon of initially hot water freezing before cooler water, occurs only when the water supercools and the cooler water has a lower nucleation temperature than the warmer water"
"[] when liquid water is cooled from above 0 oC, it often will not begin freezing until it has supercooled to several degrees below 0 oC. This is why hot water can freeze before cooler water when all experimental conditions are identical except for the initial temperatures of the water. Hot water will freeze before cooler water only when the cooler water supercools, and then, only if the nucleation temperature of the cooler water is several degrees lower than that of the hot water. Heating water may lower, raise or not change the spontaneous freezing temperature."

The Brownridge paper shows that there is no need for exotic theories about OH bonds. You either have different (time-dependent) boundary conditions in the hot and cold experiment, or your water is actually different (different number of nucleation sites).
 

russ_watters

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I agree, there is indeed no paradox, but the Mpemba effect is real nonetheless. Have you read the Brownridge paper referred to above? I think it addresses most issues we are discussing. He performed detailed measurements (in an actual lab) and he could create setups where the Mpemba effect never occurs and setups where it always occurs.
I hadn't been able to access it until now. Here's the corrected link (and I fixed it in its original post as well):
https://www.binghamton.edu/physics/docs/Preprint and Supplemental 9 Mar 10.pdf

I'm only 7 pages into it, but so far it is an excellent article. It says what I and others have been saying: if all other conditions but starting temperature are controlled to, there is no effect, but the effect can be generated by introducing certain inequalities. The first one (non-control) is where two copper cups are placed on a bed of freezer frost. The warm one melts the frost, creating a vastly improved heat transfer interface and much faster cooling.
One issue is that 'everything else equal' (which is by the way not a requirement in the description of the Mpemba effect...
Disagree. If "everything else is equal" is not at least implied, then there is no mystery/paradox to resolve. I could easily just say "hot water freezes faster than cold water if I put the hot water in the freezer and drink the cold water." That's not just unsurprising and not a paradox, it's just plain stupid. To put a finer point on it, the OP says "identical" and the wiki on the subject in different descriptions uses "identical" and "similar" and in folk tales if a difference were known one would do better to directly exploit the actual difference rather than indirectly exploit it by using hot water.
Of course, by the time the hot water has reached the initial temperature of the cold water, there must be a difference, in the boundary conditions or elsewhere, between the hot water setup and the cold water setup.
That isn't true. In the first experiment in the article, he sealed the water in a glass vial and repeated the experiment dozens of times (both with tap water and de-ionized water) in order to make all other conditions besides starting temperature exactly identical. As should be expected, the effect did not manifest.

[edit]
This subject actually irritates me a little because the typical flatly stated urban myth description can lead people to wrong understanding and/or wrong actions if someone believes it at face value. Wiki credited Aristotle (oh, how I despise Aristotle) with popularizing the general practice of leaving water in the sun before trying to freeze it (where did they put it to freeze it?), which is the wrong thing to do. I can see people now blithely putting hot water in their ice trays because they need a lot of ice for a party and think that will make it freeze faster!
 
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A.T.

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if all other conditions but starting temperature are controlled to, there is no effect,
What about the experiment in post #20. Would that work with room twater too?
 

russ_watters

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What about the experiment in post #20. Would that work with room twater too?
I don't know - it wasn't demonstrated. But while it may be similar if you are correct in your prediction, it strains the description of the effect and doesn't lend itself well to quantitative measurement of what is happening.
 
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Surely someone has observed two equal containers, with equal amounts of water and at EQUAL temperature, one freezing faster than the other. Not every time, mind you, but under just the right conditions. Sounds a bit like the Mpemba effect. Maybe the higher temperature is a distraction, maybe something is happening at the quantum or a much larger level that effects freeze times.
 
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Surely someone has observed two equal containers, with equal amounts of water and at EQUAL temperature, one freezing faster than the other. Not every time, mind you, but under just the right conditions. Sounds a bit like the Mpemba effect. Maybe the higher temperature is a distraction, maybe something is happening at the quantum or a much larger level that effects freeze times.
What happens is that when people says "hot water can freeze faster than cold water" they forget that, in a *scientific* experiment and not in home made one, "water" means "pure water" and not "water that can have air, gases, particles, salts inside/dissolved" and that in a scientific experiment the conditions must be controlled, equal for the two comparisons and the envinronment must be suitable for a freezing comparison (so it's not acceptable to make the experiment with environmental air temperature below 0°C as in experiment shown in #R20, for example, because in the case of hot water, the faster vaporization and then condensation subtracts amount of substance faster; it's as if you would manually take away water from one container, it's not acceptable).

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lightarrow
 
Newton's law of cooling is based on the observed fact that heat flow is proportional to temperature difference. So if you start off with twice the temperature difference you get twice the heat flow. When the temperature of the "hot" water reaches the starting temperature of the "cold" water, it must continue following that law and follow exactly the same temperature profile that the "cold"water followed.
Thank you for explaining your statement. But, if you can, on a night when the temperature is below freezing, can you take a pot of boiling water and throw it outside? I've done this experiment. If I remember correctly, when you do the experiment, the water should turn into snow or graupel(which is a form of precipitation between hail, sleet, rain, and snow). I know it's not the same as what the question is exactly asking, but it might be cool to do with both normal water and distilled water(or pure water, distilled is close to it and easier to find) to see if their is a noticeable difference.
 
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Disagree. If "everything else is equal" is not at least implied, then there is no mystery/paradox to resolve. I could easily just say "hot water freezes faster than cold water if I put the hot water in the freezer and drink the cold water." That's not just unsurprising and not a paradox, it's just plain stupid. To put a finer point on it, the OP says "identical" and the wiki on the subject in different descriptions uses "identical" and "similar" and in folk tales if a difference were known one would do better to directly exploit the actual difference rather than indirectly exploit it by using hot water.
I was thinking about the following: if you boil water for a while, the freezing temperature of the water will change, so the initial composition of the water is different. Is this violating the conditions of 'all else equal'? The two water samples were the same before you started boiling one sample.
Actually, if boiling the water would add nucleation agents to the water, then the freezing temperature would be higher than the non-boiled water and the Mpemba effect could be due to this. But the opposite (less nucleation agents when you boil the water) is happening as shown in the paper, so boiling for a long period of time and cooling a sample of this water should even increase the time of freezing compared to a sample that was not boiled!

That isn't true. In the first experiment in the article, he sealed the water in a glass vial and repeated the experiment dozens of times (both with tap water and de-ionized water) in order to make all other conditions besides starting temperature exactly identical. As should be expected, the effect did not manifest.
Yes. My point was that something (a boundary condition, the composition of the water, the convection current in the water) must be different (at the moment that the hot water reaches the temperature of the cold water) or else the effect cannot occur. I think that the experiment with the copper containers in Brownbridge fulfills the requirement 'all else equal at the start of the measurement'.

The convection currents in the water might also play a dominant role under certain circumstances, but I haven't seen many studies on this, only this one on the royal society of chemistry website:
http://www.rsc.org/images/adam-smith-paper-entry_tcm18-225152.pdf [Broken]
According to these measurements, if your initial temperature is around 5 C then no convection currents will be formed. When you start with a higher temperature (20,40,60,80,100), the convection currents will result in a stronger cooling rate. There is then a trade-off between the time it takes to cool down from the initial temperature and the increase in cooling rate that you will (eventually) get. Also, the exact moment that the water starts to freeze is a statistical event and they show quite some spread in their measurements for the 5 C samples.
 
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sophiecentaur

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(where did they put it to freeze it?
I believe they used shallow ponds which freeze over during cold desert nights. The ice was skimmed off and taken to cold caves with straw insulation. That way, wealthy Arabs could have iced drinks at noon in the desert. What a status symbol! Added salt could give them a freezing mixture.
 
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I believe they used shallow ponds which freeze over during cold desert nights. The ice was skimmed off and taken to cold caves with straw insulation. That way, wealthy Arabs could have iced drinks at noon in the desert. What a status symbol! Added salt could give them a freezing mixture.
Where did the Arabs come from? In Aristotle?
I tried to see what he did actually said. It's not easy because the references indicate just the book not any section or page (they did not have pages in their scrolls, I suppose),]
In his book "Meteorology" I found (Book 1, part 12) the claims that people heat water in the sun so it cools down faster. He does not explain who does it and where the idea comes from.
And also mentions the practice of the inhabitants of the Pontus to pour hot water on their reeds when they go ice fishing. The claim is that it will freeze faster.
Does not say if he really went there or just heard it from storytellers. :smile:
Pontus is somewhere is today Turkey but at that time there were no Arabs (and no dessert) over there.
Maybe he mention Arabs somewhere else.
 

A.T.

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" The behaviour seems contrary to natural expectation but many explanations have been proposed.
wikipedia said:
  • Evaporation: The evaporation of the warmer water reduces the mass of the water to be frozen. Evaporation is endothermic, meaning that the water mass is cooled by vapor carrying away the heat, but this alone probably does not account for the entirety of the effect.
  • Convection: Accelerating heat transfers. Reduction of water density below 4 °C (39 °F) tends to suppress the convection currents that cool the lower part of the liquid mass; the lower density of hot water would reduce this effect, perhaps sustaining the more rapid initial cooling. Higher convection in the warmer water may also spread ice crystals around faster.
  • Frost: Has insulating effects. The lower temperature water will tend to freeze from the top, reducing further heat loss by radiation and air convection, while the warmer water will tend to freeze from the bottom and sides because of water convection. This is disputed as there are experiments that account for this factor.
  • Supercooling: It is hypothesised that cold water, when placed in a freezing environment, supercools more than hot water in the same environment, thus solidifying slower than hot water. However, super-cooling tends to be less significant where there are particles that act as nuclei for ice crystals, thus precipitating rapid freezing.
  • Solutes: The effects of calcium carbonate, magnesium carbonate among others.
  • Thermal conductivity: The container of hotter liquid may melt through a layer of frost that is acting as an insulator under the container (frost is an insulator, as mentioned above), allowing the container to come into direct contact with a much colder lower layer that the frost formed on (ice, refrigeration coils, etc.) The container now rests on a much colder surface (or one better at removing heat, such as refrigeration coils) than the originally colder water, and so cools far faster from this point on.
  • Dissolved Gases: Cold water can contain more dissolved gases than hot water, which may somehow change the properties of the water with respect to convection currents, a proposition that has some experimental support but no theoretical explanation. "
source: https://en.wikipedia.org/wiki/Mpemba_effect#Suggested_explanations
 
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sophiecentaur

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Where did the Arabs come from? In Aristotle?
I was making a point that the technology was there, long before Electrolux.
But your doubt about the experimental evidence is probably well founded. I understand that a lot of Greek Science was based on simple observation without actually setting up experiments. I believe that it was not until the middle ages that the Scientific method actually required exhaustive experimentation so Aristotle may well have relied on hearsay.
That binghampton paper seems to be a pretty competent bit of work and a lot more believable than statements from the ancient past.
 
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Think about it. Boiling water evaporates very fast.It could have evaporated more than the room temperature one and have to absorb less heat.Some may argue that the difference would not have affected that much but remember, the water in two containers AREN'T the EXCAT same. The ions? The volume? If a more controlled experiment is done,I'm sure a different result will be achieve.
 
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And also evaporation takes heat away and there is no way to stop that so we can say hot water 'cheated'
 
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I haven't read all the posts nor I am an expert but if we think in this way that water is a strange liquid.
1. It expands anomalously and therefore the Mpemba effect might be correct.
2. On freezing, water molecules arrange themselves in hexagonal structures with hollow space in between them. I think that on heating the water molecules move more freely and so they are easily able to arrange themselves in this structure form.
3. And yes I do believe that larger surface area/volume ratio of hot water also contributes in rapid heat loss from it.
 
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And also evaporation takes heat away and there is no way to stop that so we can say hot water 'cheated'
Put the two samples into sealed containers instead of open ones?
(Best would something like clear plastic so that fracturing can't happen due to pressure or ice, but you can still see what is happening.)
 

sophiecentaur

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If a more controlled experiment is done,I'm sure a different result will be achieve.
Did you read all the links in this thread? There have already been many investigations and the results have been very dependent on the conditions and it depends on how you determine the 'freezing' has taken place.
 

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