Scientists offer prize for explanation of Mpemba effect

[Heisenberg]

http://www.rsc.org/AboutUs/News/PressReleases/2012/mpemba-effect-water-ice-hot.asp

If you think you can figure out this mystery which has frustrated science for years, you can win.....$1600.



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.

http://en.wikipedia.org/wiki/Mpemba_effect

 

[maartards]

damn lets just grab a bowl and figure it out lol

 

[MajorCoco]

I heard about this problem a few years ago. It annoyed me at the time, because I found out about it after having very carefully explained to someone (who was telling me about it!) why they were wrong...only to find google disagreeing with my flawless logic. I like to think I'm a know-it-all, so being out-done was a painful experience!! hehe

My argument went something like this: It is impossible for warm water to freeze faster then cold, because at some point in its progress to being frozen it must, by definition, overtake the temperature of the originally cold water, at which point we would be able to say "Why would two beakers of water at the SAME temperature" freeze at different times?"

When you look at it from this perspective, it becomes clear that there must be something else going on. (or the tests are wrong and it doesn't actually happen...think how many times people have "proved" that they have crossed the equator by watching water going down a plughole, when in fact the effect is basically random)

Assuming that the test were properly and conclusively done, I have 2 main theories:
- The warm water, placed into cold conditions, may occasionally find that it turns into a "driven dynamic fluid system", where the temperature differential between the warm water and it's cold environment will create stable convection currents. If this happened, then you might expect that this would more efficiently extract heat from the fluid, freezing it faster. This could happen if one side of the glass were being cooled more than the other side.
- The 2 fluids being compared may be physically different. We all know that cold water holds more O2. Perhaps the action of heating the water, reducing the O2 concentration among other things, could be a factor.

I have no idea if either theory is any good, and I studied physics at university!! I'll have another think about it this evening over a spliff. See if I can come up with anything else...

 

[MajorCoco]

Haha...just went to wiki, and it seems both my ideas are old, and disproved....back to the drawing board

 

[Heisenberg]

Haha...just went to wiki, and it seems both my ideas are old, and disproved....back to the drawing board

I still enjoyed reading them. The controversy behind the Mpemba effect has always struck me as an example of the scientific method wrestling with itself. Will be interesting to see the outcome.

 

[OGEvilgenius]

Well, colder water is denser and behaves quite differently in terms of chemical reactions, does/is it not? And water has a high specific heat capacity. Also frozen water is less dense than liquid due to the molecular structure. I would guess that the two things are closely related. It's like taking a few molecules spaced out and easily having cold overtake each individual - and because of what happens to water at a molecular level when frozen, water that is heated is actually closer to that state and it is easier to effect the structure as there is more molecular surface area in hot water than there is in cold water. Colder water acts like insulation more so and hot water acts less like insulation. I'm not an expert in any of this, just going by what I remember from high school chemistry.

 

[1337hacker]

I like where you are going with this. We should freeze up some waters at various temperatures and check when exactly , and how long the freezing stage is lasting, and the rate of cooling that is occuring on it's way to a freezing point. I bet that data would help a lot.

 

[mipainpatient]

Not sure if this helps but last winter I went out to my garage and noted that I had two pyrex bottles of water, on shelves a couple of feet from one another. The top bottle was frozen solid, and the bottom one appeared to be liquid. I grabbed the bottle up and felt it, sure enough, it was quite cold. I swished it around lazily, not really sure what I was thinking about as I did it. I was puzzled to notice that it seemed thick, more viscous than water normally is. I set it down and abruptly noticed that it was no longer clear, snowflake-like structures began to form and the liquid had turned to slush at the top of the container. I shook the container for a bit more, realizing that I had initiated some sort of "order" in the bottle and it froze solid in seconds!
Seemed relevant....
Correct me if I speak wrong but this was super-cooled water? Didn't have enough energy to freeze but was clearly below freezing temperature?

 

[BA142]

As you try to freeze the warm water a bit of it is bound to evaporate leaving less volume...maybe that's why?

 

[jessy koons]

Interesting problem but is $1600 really going to motivate our best thinkers?

 

[Heisenberg]

Not sure if this helps but last winter I went out to my garage and noted that I had two pyrex bottles of water, on shelves a couple of feet from one another. The top bottle was frozen solid, and the bottom one appeared to be liquid. I grabbed the bottle up and felt it, sure enough, it was quite cold. I swished it around lazily, not really sure what I was thinking about as I did it. I was puzzled to notice that it seemed thick, more viscous than water normally is. I set it down and abruptly noticed that it was no longer clear, snowflake-like structures began to form and the liquid had turned to slush at the top of the container. I shook the container for a bit more, realizing that I had initiated some sort of "order" in the bottle and it froze solid in seconds!
Seemed relevant....
Correct me if I speak wrong but this was super-cooled water? Didn't have enough energy to freeze but was clearly below freezing temperature?

[video=youtube;CzEC7WWeNyA]http://www.youtube.com/watch?v=CzEC7WWeNyA[/video]

You are not wrong, no correction needed.

http://en.wikipedia.org/wiki/Supercooling

 

[Heisenberg]

As you try to freeze the warm water a bit of it is bound to evaporate leaving less volume...maybe that's why?

Good idea, but when we perform experiments controlling for evaporation, the effect still occurs.

Interesting problem but is $1600 really going to motivate our best thinkers?

I was thinking the same thing, but I suppose it's enough to get the idea out there to the press and people. The prestige would be worth quite a lot to some as well.

 

[cannabineer]

Not sure if this helps but last winter I went out to my garage and noted that I had two pyrex bottles of water, on shelves a couple of feet from one another. The top bottle was frozen solid, and the bottom one appeared to be liquid. I grabbed the bottle up and felt it, sure enough, it was quite cold. I swished it around lazily, not really sure what I was thinking about as I did it. I was puzzled to notice that it seemed thick, more viscous than water normally is. I set it down and abruptly noticed that it was no longer clear, snowflake-like structures began to form and the liquid had turned to slush at the top of the container. I shook the container for a bit more, realizing that I had initiated some sort of "order" in the bottle and it froze solid in seconds!
Seemed relevant....
Correct me if I speak wrong but this was super-cooled water? Didn't have enough energy to freeze but was clearly below freezing temperature?

I would say yes. i have seen lakes supercool at the surface. When i pitched a pebble in, a "collar" of surface ice sprang into being from the nucleation by turbulence.

I've also had seltzer water sitting in a slightly subfreezing fridge ... I pulled one out and watched immediate feathers of ice growing. Cool. Figuratively, also. cn

<edit> One thing worth remembering about the "frozen beer" etc. is that the degree of supercooling is mild, a few degrees. The water-to-ice transition is exothermic, with a heat of 80 calories/ml ... This means the water would have to be at about minus eighty to freeze truly solid. The ice crystals are feathery and form an apparently solid slush that is actually 5% or less solid. Since water expands on freezing, and since the fill level in the bottle didn't change noticeably (and the bottle didn't explode, lol) I conclude that the final percentage of ice is low.

 

[Doer]

Well, colder water is denser and behaves quite differently in terms of chemical reactions, does/is it not? And water has a high specific heat capacity. Also frozen water is less dense than liquid due to the molecular structure. I would guess that the two things are closely related. It's like taking a few molecules spaced out and easily having cold overtake each individual - and because of what happens to water at a molecular level when frozen, water that is heated is actually closer to that state and it is easier to effect the structure as there is more molecular surface area in hot water than there is in cold water. Colder water acts like insulation more so and hot water acts less like insulation. I'm not an expert in any of this, just going by what I remember from high school chemistry.

I saw it on Mythbusters and it is really quite simple. It can be proved very easily. I think $1600 is a joke. That's because, there is no unproven Mpemba effect. Ice is an insulator. If you allow the to water insulate itself, it will slow down the freeze a great deal.

If it is warmer then it creates the convection plume that carries the heat up, and will freeze the water from the bottom. So what?

 

[Doer]

I like where you are going with this. We should freeze up some waters at various temperatures and check when exactly , and how long the freezing stage is lasting, and the rate of cooling that is occuring on it's way to a freezing point. I bet that data would help a lot.

The easy way to do this is on a window ledge in winter. Use an eye dropper. Why? Drops are indentical enough in vol. Drops of water will allow for more fun per min.

The hot drop will freeze so much faster it is amazing.

 

[Seedling]

The easy way to do this is on a window ledge in winter. Use an eye dropper. Why? Drops are indentical enough in vol. Drops of water will allow for more fun per min.

The hot drop will freeze so much faster it is amazing.

Hence the reason why hot drops will freeze faster than cold drops, when the VOLUME of the drops is the same, the hot drops are LESS MASSIVE, which means they have less matter. It's the density that matters, not the temp. Given the same volume of two different massive objects of the same material, the LESS DENSE object will reach a state of lower potential energy in less time than the more dense object to that same potential energy state, because the less dense object starts at a lower potential energy to begin with, and entropy increases at the same rate, according to the second law of thermodynamics.

To further this idea of a less dense object reaching a lower potential energy state than a more dense object in the same amount of time, let's keep the same MASS of water, hence the hotter mass will be a greater volume, again, less dense. The hotter less dense water (think of it as steam) will freeze much faster than the same mass of water with a lower volume (more dense).

 

[cannabineer]

Hence the reason why hot drops will freeze faster than cold drops, when the VOLUME of the drops is the same, the hot drops are LESS MASSIVE, which means they have less matter. It's the density that matters, not the temp. Given the same volume of two different massive objects of the same material, the LESS DENSE object will reach a state of lower potential energy in less time than the more dense object to that same potential energy state, because the less dense object starts at a lower potential energy to begin with, and entropy increases at the same rate, according to the second law of thermodynamics.

To further this idea of a less dense object reaching a lower potential energy state than a more dense object in the same amount of time, let's keep the same MASS of water, hence the hotter mass will be a greater volume, again, less dense. The hotter less dense water (think of it as steam) will freeze much faster than the same mass of water with a lower volume (more dense).

The difference in mass (density, assuming invariant drop volume) from temperature is tiny; about four per cent. I'd expect a larger variation due to heat-induced changes in surface tension. Also, since drops have high surface to volume, I'd expect evaporation/diffusion effects to dominate, whereas in a larger volume they become negligible. cn

 

[Seedling]

The difference in mass (density, assuming invariant drop volume) from temperature is tiny; about four per cent. I'd expect a larger variation due to heat-induced changes in surface tension. Also, since drops have high surface to volume, I'd expect evaporation/diffusion effects to dominate, whereas in a larger volume they become negligible. cn

So given the same volume of iron and aluminum at the same temperature, the aluminum being less dense of course, which would you expect to first reach a room temperature of 70 degrees, if both were placed in the room at a temperature of 100 degrees? Can you name an example of a more dense object cooling at a greater rate than a less dense object, given the same volume and temperature?

 

[cannabineer]

So given the same volume of iron and aluminum at the same temperature, the aluminum being less dense of course, which would you expect to first reach a room temperature of 70 degrees, if both were placed in the room at a temperature of 100 degrees? Can you name an example of a more dense object cooling at a greater rate than a less dense object, given the same volume and temperature?

It's a matter of specific heat.
Take a volume of tungsten v. the same volume of liquid water. Tungsten is 19x as dense, but the ratio of specific heats (0.136 J/g K for tungsten; 4.186 for water) is 32.7. So the equal volume of tungsten can only hold 60% the heat per degree of the water. Given the same shape, the tungsten will cool faster, despite weighing nineteen times as much. cn

 

[Seedling]

It's a matter of specific heat.
Take a volume of tungsten v. the same volume of liquid water. Tungsten is 19x as dense, but the ratio of specific heats (0.136 J/g K for tungsten; 4.186 for water) is 32.7. So the equal volume of tungsten can only hold 60% the heat per degree of the water. Given the same shape, the tungsten will cool faster, despite weighing nineteen times as much. cn

Are you saying that a cubic centimeter of aluminum at a temperature of 100 degrees will cool slower than a cubic centimeter of iron at 100 degrees? Given the two metals are the same temperature at the start, with the same volumes, which will cool to room temperature faster if placed in the room which is 70 degrees? Are you saying the iron will reach 70 degrees first?