Why is the reason for "Mpemba effect" (warm water can freeze faster than cold) still a mystery?

[astro]

Why is the "Mpemba effect" still a mystery. Seems it ought to be easily explainable.

The Mpemba effect is the observation that warmer water can sometimes freeze faster than colder water. Although the observation has been verified, there is no single scientific explanation for the effect.

[Francis Vaughan]

Clearly the SDMB needs to put in an entry for the thousand pound prize for this question.

[voltaire]

Originally Posted by astro

Why is the "Mpemba effect" still a mystery. Seems it ought to be easily explainable.

The wiki does a decent job of addressing why explaining it isn't easy:

According to an article by Monwhea Jeng: "Analysis of the situation is now quite complex, since we are no longer considering a single parameter, but a scalar function, and computational fluid dynamics (CFD) is notoriously difficult."
<snip>
[A reviewer for Physics World] pointed out that investigations of the phenomenon need to control a large number of initial parameters (including type and initial temperature of the water, dissolved gas and other impurities, and size, shape and material of the container, and temperature of the refrigerator) and need to settle on a particular method of establishing the time of freezing, all of which might affect the presence or absence of the Mpemba effect. The required vast multidimensional array of experiments might explain why the effect is not yet understood.

My bet is on the phenomenon being due to the combined small effects of various different factors, some of which were also well-covered in the wiki.

[Crazyhorse]

Originally Posted by voltaire

The wiki does a decent job of addressing why explaining it isn't easy:My bet is on the phenomenon being due to the combined small effects of various different factors, some of which were also well-covered in the wiki.

Originally Posted by Francis Vaughan

Clearly the SDMB needs to put in an entry for the thousand pound prize for this question.

Anyone catch the cite at the bottom of the Wiki? (I knew this question sounded familiar from somewhere)

Adams, Cecil; Mary M.Q.C. (1996). "Which freezes faster, hot water or cold water?". The Straight Dope. Chicago Reader, Inc. Retrieved January 2008.

[Senegoid]

Thanks, crazyhorse, I just knew there was a Cecil column on this.

I feel almost sure I've also seen this discussed with respect to the business of resurfacing ice skating rinks with the famous Zamboni -- should you scrape the ice and spray it with cold water or warm water? But, sorry, now I can't find anything on that. (I though the Cecil column was about ice resurfacing, but upon review, it isn't.)

[Chronos]

Has anyone even established that this happens at all, under controlled conditions?

[astro]

Originally Posted by Chronos

Has anyone even established that this happens at all, under controlled conditions?

You're a physicist. Why don't you bust this mystery open, claim the 1000 pound prize and get your name in the history books.

[jebert]

This makes no sense. at least from a theoretical point of view. Say you start with two samples of water, one at 90 degrees and one at 50 degrees. Put the 90 degree water into the freezer and track its temperature. When it gets to 50 degrees, put the other sample in. Now you have two samples at the same temperature, which one would think would take the same time to freeze. Obviously the hotter water took a finite time to cool to the temperature of the cooler water. So the hotter water had to take longer to freeze. No?

[Greg Charles]

Originally Posted by jebert

This makes no sense. at least from a theoretical point of view. Say you start with two samples of water, one at 90 degrees and one at 50 degrees. Put the 90 degree water into the freezer and track its temperature. When it gets to 50 degrees, put the other sample in. Now you have two samples at the same temperature, which one would think would take the same time to freeze. Obviously the hotter water took a finite time to cool to the temperature of the cooler water. So the hotter water had to take longer to freeze. No?

Eminently logical. However, both Wikipedia and Cecil make clear that the effect is observed between water very close to boiling point (100 C) and water a few degrees cooler than that. 50 degree water will definitely freeze faster than 90 degree water, whether you mean F or C.

[dracoi]

Originally Posted by Greg Charles

Eminently logical. However, both Wikipedia and Cecil make clear that the effect is observed between water very close to boiling point (100 C) and water a few degrees cooler than that. 50 degree water will definitely freeze faster than 90 degree water, whether you mean F or C.

Well...what I'm reading right now in Wikipedia is this:

If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first.

And later:

New Scientist recommends starting the experiment with containers at 35 °C (95 °F) and 5 °C (41 °F) to maximize the effect.

So Wikipedia does not agree with Cecil.

[Greg Charles]

Hey, you're right! I saw the 95 and misread it as 95 C. Stupid units!

I'm with Cecil. It's absurd that 35 degree water would freeze faster than 5 degree water. I don't believe it, even though Wikipedia seems to cite reliable sources. Wikipedia weasels a bit by quoting one source that says "sometimes" it could happen that way, but the reference is to a dead link.

[steviep24]

Hot water pipes in a plumbing system are more likely to freeze and burst before cold water pipes so there is some truth to the Mpemba effect.

[billfish678]

Originally Posted by astro

You're a physicist. Why don't you bust this mystery open, claim the 1000 pound prize and get your name in the history books.

This proves one thing.

Those guy's offering the prize are a bunch of cheap SOB's.

[Mangetout]

Originally Posted by jebert

This makes no sense. at least from a theoretical point of view. Say you start with two samples of water, one at 90 degrees and one at 50 degrees. Put the 90 degree water into the freezer and track its temperature. When it gets to 50 degrees, put the other sample in. Now you have two samples at the same temperature, which one would think would take the same time to freeze. Obviously the hotter water took a finite time to cool to the temperature of the cooler water. So the hotter water had to take longer to freeze. No?

That makes sense if you expect the samples to be homogenous at all points in the process, but is that really the case?

Are all properties of a container of water that has just been heated to 50 celsius the same as the properties of a similar container of water that has been boiled, then left alone to cool to 50 celsius in a freezer? Probably not.

[TriPolar]

I just tested this. I put a gallon of water at 72F in a freezer alongside 1 ounce of boiling water, approximately 212F. The hot water froze solid before the cold water did. It's because the ratio of surface area to volume is greater in the hot water because it's a much smaller volume allowing the heat to dissipate more quickly. A noticeable amount of the boiling water evaporated before it froze also. Where's my £1000 prize?

[Lumpy]

[tongue in cheek]It's because water has a "memory", in which it's previous states affect it's current behavior. It's the basis for homeopathy and polywater.[/tongue in cheek].

The scary thing is, some experiments act like this is true!

[Really Not All That Bright]

Originally Posted by Greg Charles

Hey, you're right! I saw the 95 and misread it as 95 C. Stupid units!

I'm with Cecil. It's absurd that 35 degree water would freeze faster than 5 degree water. I don't believe it, even though Wikipedia seems to cite reliable sources. Wikipedia weasels a bit by quoting one source that says "sometimes" it could happen that way, but the reference is to a dead link.

I think you're still having problems with your units. Water at 5 degrees Fahrenheit is already frozen.

[RearEchelon]

Might it have something to do with the fact that the molecules in a sample of warm water have more energy and can therefore give up that energy faster? IANAPhysicist, so this may sound ridiculous to some of you, but it's just a thought.

[AaronX]

Originally Posted by Mangetout

That makes sense if you expect the samples to be homogenous at all points in the process, but is that really the case?

Are all properties of a container of water that has just been heated to 50 celsius the same as the properties of a similar container of water that has been boiled, then left alone to cool to 50 celsius in a freezer? Probably not.

One thing that might be different is stirring, i.e. water cooled to 50C is in convection, while water starting from 50C is still. Once water freezes, starting from the outside, it's harder to freeze the inside if it's not stirred, so it takes longer.

[dnooman]

Primarily because the conditions and all of the variables therein, vary so much as to cause different results when lay-people test them.

You first have to eliminate the variable of a freezer. They act differently when cold and very hot water are frozen, partly due to the freezer cycling when hot water tells the freezer thermostat that it needs to cool again.

Also, you need to weigh both samples before putting it into a freezing situation and after, mass may be lost due to evaporation of the hotter water.

And what defines frozen? Ice on the top, or it being frozen solid?

I'd like to see this test done outside in a very cold area, like Norway or Siberia at extremely low temps. That eliminates the mechanical aspect that affects most people freezing water. Weigh both samples before they are introduced to the cold, then weigh them after they have been frozen solid for starters. If there's less mass, there's less to freeze right?

Common thinking says that hotter water has to cool farther than colder water in order to reach the freezing point of water. But, if certain scenarios prove otherwise, then there may be something on a molecular level that allows heated water (and it's excited electrons) to behave differently that those of cooler water when a state change is involved.

Ultimately, the Mpemba effect is still a mystery because a) it can't be reliably reproduced at home, and b) the physics of "water at STP at x degrees C will always freeze at this rate" has never been proven. I suspect molecular excitation has at least something to do with the issue, but obviously IANAP.

Who knows? Barometric pressure and humidity may even factor in. Perhaps the reason it hasn't been researched to the point of conclusion, is that there's no grant money available for such a thing?

My uninformed opinion is that such studies should take place in sub-freezing temperatures, timed and weighed at every possible temp level and evaluated for when the core hits 0 degrees C.

[dnooman]

Sorry, upon re-read of the OP you were asking why it's unsolved, not why lay-people don't get it.

If leading physicists can't solve it, they will certainly not be asking me for answers.

[scm1001]

Water will not freeze straight away at 0 C. It will usually supercool before solidifying- the amount of supercooling will depend on things like the amount of dissolved gas - water that has been heated will contain less dissolved gas - this may make a key difference. If the rate of supercooled ice formation is slower than the rate of cooling then this may be the answer to the problem.

E.g it might take 5 minutes for a 40 C water sample to cool to -4 in the freezer and 10 minutes for a 90 C water sample. But if ice formation took 30 minutes due to supercooling, then that effect wins out.

[Chefguy]

Originally Posted by steviep24

Hot water pipes in a plumbing system are more likely to freeze and burst before cold water pipes so there is some truth to the Mpemba effect.

Do you have a cite for this, because I'm not buying it.

[Quercus]

Originally Posted by dnooman

then there may be something on a molecular level that allows heated water (and it's excited electrons) to behave differently that those of cooler water when a state change is involved.

I don't think we need to start revising basic physics, yet. As the wiki article states, there are a lot of possible explanations that are perfectly compatible with physics. I think evaporation and convection are most likely.

[Greg Charles]

Originally Posted by Really Not All That Bright

I think you're still having problems with your units. Water at 5 degrees Fahrenheit is already frozen.

That'd also be true if I meant 5 Kelvin, and if I meant a 5 degree angle, it wouldn't make sense at all.

Why would you assume I was talking about degrees Fahrenheit?

[Dufus]

My physics book says that heat flows from hot to cold. The greater the temperature difference, the faster the heat flows. The sample of hot water placed in a freezer looses its heat faster than the cold water. The temperatures rapidly equalize and the two samples freeze at the same time.

Another mystery to ponder is if the question stumped Aristotle, Roger Bacon, Francis Bacon, Descartes and all the best brains for the last 2,300 years, why is it named after a school child who asked about it in 1968?

[TriPolar]

I'm still waiting to hear about a mystery. The only way hot water freezes faster than cold water is when the conditions are different. So hot pure water freezes faster than cold salt water, hot water that evaporates rapidly so that a much smaller volume of water is left to freeze will freeze faster than cold water at the original volume, hot water in a metal container will freeze faster than cold water in an insulated container. There's no mystery there.

I am offering my own 1000 pound prize to anyone who can show that hot water freezes faster than cold water using the same amount of water by weight in sealed containers of the same type in the same freezer conditions. Wait... what's 1000 pounds in real money? ... let's see ... 30 days hath september ... carry the one ... oh yeah, I can afford that. Not like I'll have to. But to keep things reasonable, the volume of water has to be a least 1 cubic inch, and the freezer kept at no lower than 0F, and the container made out of common materials.

[Frylock]

Will boiling water melt a ziploc bag?

[Arkcon]

Originally Posted by Dufus

Another mystery to ponder is if the question stumped Aristotle, Roger Bacon, Francis Bacon, Descartes and all the best brains for the last 2,300 years, why is it named after a school child who asked about it in 1968?

Because its not a physics problem. Its an ideological parable with an axe to grind. Mpemba stated an observation that he knew well, and no one believed him. Often, people bring up the Mpemba effect, just to say that science can't explain everything. It's already happened in this thread.

My pure wag ... I assume means wild arbitrary guess: hot, not boiling, water, in a wooden bucket, will ice over on top, faster than a similar bucket of colder water, when left outside at sub freezing temperatures. Because the evaporative cooling cools the surface of the water, while the wooden bucket insulates both well enough to keep the interior from cooling quickly. Those are the conditions Aristotle, Roger Bacon, Francis Bacon, and very likely Descartes were working under. None of them had cycling refrigerators, thermocouple temperature probes, plastic containers, distilled water, and most of them were working without the scientific method.

[filmore]

I've heard the difference is because the hot water is evaporating more than the cool water. So the hot sample may freeze faster, but it ends up being smaller.

But it seems you could easily eliminate the evaporation variable. Take two plastic soda bottles and pour the same amount of water in each with some air space at the top and seal them up. Put one bottle in the microwave for a short while to heat up the water. Put both bottles in the freezer and see which freezes first.

[california jobcase]

I have heard from people who heard from plumbers that hot water pipes freeze more often than cold water pipes. However, I contend the water in hot water pipes when they freeze is previously heated water that was at the same temperature as the water in the cold water pipes long before the pipes froze.

Even on a warm day, one generally has to wait a bit for the hot water from the water heater to reach the spigot. The idea that the water in hot water pipes is always hot is false.

[astro]

Originally Posted by Frylock

Will boiling water melt a ziploc bag?

No. I freeze soups in zip locks and then boil them in the same bags all the time. The heat and superheated steam of the food inside the bag will usually (most of the time) open up or break the zip lock seal when the soup is boiling but the plastic isn't "melting".

[The Great Sun Jester]

Originally Posted by Dufus

My physics book says that heat flows from hot to cold. The greater the temperature difference, the faster the heat flows. The sample of hot water placed in a freezer looses its heat faster than the cold water.

So, the property of heat loss has momentum?

Just foolin'. I deliberately edited out "The temperatures rapidly equalize and the two samples freeze at the same time" because I am a douchenozzle.

[Dufus]

Apparently, Erasto Mpemba was turning the handle of an ice cream maker. He noticed that boiling hot ice cream mix froze up and solidified faster than ice cream mix that had cooled to room temperature. In order to reproduce this at home and claim TriPolar's money, we need to find out what kind of ice cream it was, and get to cranking.

[YWalker]

Way back when in my college physics class, my professor pointed out that radiation heat loss is proportional to the absolute temperature (in Kelvin) of the first surface raised to the fourth power minus the absolute temperature of the second raised to the fourth power. As a result, the rate of heat loss from the higher temp water is significantly higher than from the colder.

[AaronX]

Originally Posted by filmore

Put one bottle in the microwave for a short while to heat up the water. Put both bottles in the freezer and see which freezes first.

Do not heat a sealed container.

[TriPolar]

Originally Posted by YWalker

Way back when in my college physics class, my professor pointed out that radiation heat loss is proportional to the absolute temperature (in Kelvin) of the first surface raised to the fourth power minus the absolute temperature of the second raised to the fourth power. As a result, the rate of heat loss from the higher temp water is significantly higher than from the colder.

Just because this point keeps coming up, what do you suppose happens after the hot water cools to the same temperature as the cold water?

[NinetyWt]

Originally Posted by california jobcase

I have heard from people who heard from plumbers that hot water pipes freeze more often than cold water pipes. However, I contend the water in hot water pipes when they freeze is previously heated water that was at the same temperature as the water in the cold water pipes long before the pipes froze.

Even on a warm day, one generally has to wait a bit for the hot water from the water heater to reach the spigot. The idea that the water in hot water pipes is always hot is false.

Also, I bet the cold water is used more often; therefore, the water in the 'cold water' pipes moves more often than the water in the 'hot water' pipes. Still water freezes faster than moving water.

[user_hostile]

Would gravity have any influence (i. e., convection)? I wonder if performing the experiments in zero-G would narrow the list of possible causes. Anybody know an astronaut on the ISS?

[Greg Charles]

Let's say we wanted to perform this experiment in our own freezers. How do we determine when water is frozen? Poke it with a toothpick? In ice trays, water usually freezes around the edges first, right? Do we have to establish that it's frozen solid for it to be a valid test? Is opening the freezer door every couple of minutes and poking the samples with toothpicks going to mess up the results?

[filmore]

If you compare the composition of water before and after it is boiled, it will be different. To make coffee you should start with cold water rather than hot or previously boiled water since they will have different tastes. So the difference in freezing temperatures could be because the composition of boiled water is different than the original sample.

A test for this would be to have two water samples. Boil one and let it cool back to room temperature. Then freeze it along with the unboiled sample and see if one freezes first.

[Greg Charles]

Yes, that makes some sense. However, the claim is warm, unboiled water also freezes faster than cool water. (35 degrees C! vs. 5). Also, Cecil failed to duplicate the effect even when he use boiled water that had cooled:

Originally Posted by Cecil Adams

The cold water froze about 10 or 15 minutes faster than the hot water, and there was no detectable difference between the boiled water and the other kind.

[AdamF]

Originally Posted by TriPolar

Just because this point keeps coming up, what do you suppose happens after the hot water cools to the same temperature as the cold water?

One problem with this statement is that the average temperature of water has little effect on the cooling rate. So you can have two samples, both with average temp of say 15 C. But one of them has surface temp of 30 C and will cool faster than the other one with surface temp of only 20 C.

You should read the original paper, it is linked on the wiki page.

[TriPolar]

Originally Posted by AdamF

One problem with this statement is that the average temperature of water has little effect on the cooling rate. So you can have two samples, both with average temp of say 15 C. But one of them has surface temp of 30 C and will cool faster than the other one with surface temp of only 20 C.

You should read the original paper, it is linked on the wiki page.

I think that is nonsense. For the water to freeze it must all drop to 0C. Now if you're talking about an open container made of material that has some insulating properties, the top may freeze first, changing the pattern of freezing, but unless there is a loss of water due to evaporation, that won't help. And I suspect (but don't know), that having the surface freeze first in any circumstance will slow down the freezing process.

[Crab Rangoon]

My physical chemistry professor explained it succinctly and clearly:

It is an old wives' tale.

That is why it is still a mystery.

[AdamF]

Originally Posted by TriPolar

I think that is nonsense. For the water to freeze it must all drop to 0C.

No, water has maximum density at about 4C, which then sinks to the bottom. It is perfectly possible for ice to form on the surface of water that has average temperature 0-4 C.

Originally Posted by TriPolar

Now if you're talking about an open container made of material that has some insulating properties, the top may freeze first, changing the pattern of freezing, ...

Isn't this what we are talking about?

[TriPolar]

Originally Posted by AdamF

No, water has maximum density at about 4C, which then sinks to the bottom. It is perfectly possible for ice to form on the surface of water that has average temperature 0-4 C.

Never-the-less, it is not all frozen until the average temperature is below 0C.

Isn't this what we are talking about?

Not if you want my money.