Royal Society of Chemistry offers $1600 prize

A guy once tried to tell me that cold water boils faster than hot water. I told him he was talking nonsense. As far as I know he was. But now I see something similiar which I've never heard of before. Tepid water freezes faster than ice cold water?

It would be cool if SMP could submit a winning solution to this mystery and make 2+2 famous in Royal Society circles.


PairTheBoard

Water does seriously weird **** at surfaces. If I had to hazard a guess, I'd say that low-temperature water forms an insulating layer around the surfaces of the vessel (and the top of the liquid to a lesser degree) that higher-temperature water doesn't form allowing for better heat transfer to the environment, and as high-temperature water cools, it doesn't cool uniformly, and the thermal gradients keep enough convection going to prevent that layer from forming quickly in the formerly-hot water. I would be quite surprised if you took identical water (except for T obv) in identical vessels, both with good magnetic stirring, and the cold water didn't get "fairly frozen" faster.

I remember being told it was because there is less gas dissolved in warm water. Since it was the only explanation I've heard I assumed it must be the reason, but I guess it's not.

That would be a pretty good way to test your theory. I'm actually pretty surprised the solution hasn't been discovered yet. You'd think industry would need to know this sort of thing and some engineer would have figured it out by now. It's hard to believe £1000 is going to do the trick.

Somebody could test this at home with 2 glasses and a vibrator. (See if effect exists in the glasses alone. If so, see if effect still exists if a running vibrator is sitting on top of them)

Can you see the Mpemba Effect in other liquids too?
Not only the faster freezing but also going faster from T1 to T2.

You cant seriously try to answer this without complex physics that deals with the phase transition mechanism and how it takes place across the body of water depending on the environment and the condition of the water that in principle is different from boiled and cold. The container geometry also may play a role. Also the exact content of water matters (is it 100% water molecules, eg real drinking water never is that plus it has minerals/salts etc).

I am also willing to claim the answer depends on how fast the system is forced to freeze.

I sure hope its not a hoax as i have no personal experimental familiarity with such effect so i take it now for granted by the references. It should be a physics no chemistry problem though unless its a physical chemistry subject that chemists are more advanced in dealing than many-body/liquid state/phase transition physicists. But real material scientists are the only ones that can do it right (ie physicists) if forced to do the real problem in all its glory.


Another thing to ask is if the ice formed is identical in structure in both waters.



Something i have observed however that is different but maybe similar in explanation ultimately is that water that has been boiled and then cooled and forced to be reboiled again arrives at the rapid bubble phase without the gradual bubble phase that the originally cold water had (just get water from faucet and boil it and observe how it arrives to rapid bubbling gradually, then coolit and repeat boiling, there is always an earlier gradual slower bubbling phase before the intense one sets in in the fresh water. Of course that could be a result of the air diluted in the water having been mostly released from the original boiling.


The very first experiment i would do is take water that has been thoroughly boiled many times to remove any air and then start the experiment with this type of totally clean 100% , air free water from same cold temperature always of course and then boil and try to freeze etc . Then report and only if then we still have a difference the problem is real interesting.

Also i am ultra confident the starting temperature of both waters matters and seriously you cant expect that water that is real cold vs 90 C degree water will still lead to 90 freezing faster. The hot water seriously takes time to become cold before freezing is possible. To bypass that extra time would be ridiculous unless they both start the attempt to freeze from close enough temperatures or the coldest one is not very close to 0 anyway.


Edit: Read some of what i said (concerns for set up) and other here http://en.wikipedia.org/wiki/Mpemba_effect

Other simple thing to study is to try to start cooling down hot water and see how fast the 100C goes to say 5 C vs how fast the slightly warm 35C goes to 5 C using identical coolers. Is it a race between 100and 35 or 100 and 10 or is it a more proper race of 100 down to 20 (that period not counting) and 20 waiting for the 100 to reach 20 and then both introduced in a freezer. That last thing would be intriguing for sure if they both started from same temperature when entered in the cooler. Especially if they had similar air content,salts etc

What must be established is how intense the effect is as function of the water contents and how it changes as the starting 2 waters temperature difference changes and also how it changes if the cold water is far from 0. Also how it changes as the cooler becomes slower or very rapid in cooling ability.

Basically its all garbage without proper set up and study of what affects the phenomenon.

Supposedly it has been observed in boiled and DI water. Supposedly it's also observable, at least for some starting setups, even as faster arrival at a certain temperature (although as you point out, obviously this isn't going to hold in all setups) before anything actually freezes, so there's something going on that isn't related to phase changes (assuming all of that is true). If so, it really doesn't leave much.

Agreed.

Another thing related to boiling is that most of us i think have observed when you boil water you see the first small bubbles originate from the bottom of the vessel. Clearly when you try to boil in the warming up phase the temperature of the water is not the same across all its volume. The layers of water close to the metal surface obviously get heated high first and probably have a higher temperature during the process until all of it has reached 100 or nearby. In a cooling process similar effects on the reverse take place as the core of the water is obviously surrounded by other external layers (plus of course you also have some transport) that are the first to lose energy to the environment of the cooler on the race down.

Water, when it freezes expands, contrary to others which upon freezing contract. In effect, since the water (ice) is in an expansive state the amount of warmth needed to bring to boiling point is less than if the initial state is liquid water. Conversely, going in the opposite direction can be related to this originality of quality in water.

The expansion of water at the freezing point is a great anomaly to consideration of any type of mechanistic approach. If water didn't expand how will we have ice hockey? It's a real troubling point especially if you're a Montreal fan who can use some expansiveness to their personalities( in joke to hockey fans which probably doesn't make any sense in any case).

It would be interesting to document the quantity of evaporation of ice and liquid water in various states. I really haven't got this bull's eyed yet but the fact that water is unlike any other material in the earthly realm at or around its freezing point is significant.

Mpemba Effect
So evidently, as the 100C water cools down to 35C its condition when it reaches 35C must be different than the initial condition of the water that started at 35C. What would the difference be?


PairTheBoard

If the vessel is has non-negligible thickness, then the difference is probably a larger temperature gradient within the vessel.

I would like to see this experiment done with a sealed "thin slab" of water (this removes evaporation as a factor as well as the temperature gradient). Actually, I suspect someone has already done it. But I just don't care enough to look for it.

I'm not completely convinced this actually happens and is a robust effect...but maybe something with nucleation site density?? Though I have no idea why it would be higher for water that was recently hot.

don't think thats true.

The roller coaster method:



y->∞, The higher it goes the steeper the slope. The negative is freezing and the positive is boiling.

The higher the roller coaster starts on one side the higher (faster) it will go on (to) the other.

PairTheBoard -
"So evidently, as the 100C water cools down to 35C its condition when it reaches 35C must be different than the initial condition of the water that started at 35C. What would the difference be?"





I would think the temperature gradient (direction and rate the temperature changes the most rapidly around a particular location) at all locations within the water would be determined by the boundary temperature and the temperatures at each point within the water. I don't know how you'd do it but I'd think the distribution of temperatures throughout the water as it cools to around 35C could be experimentally determined. Once that distribution is experimentally determined for the cooling water I'd think you could artificially produce that same temperature distribution for a vessel of water starting at about 35C. Then see if it cools and freezes from then on at the same speed the initial 100C water does after it gets down around 35C. If it did it should prove that it's the peculiar temperature distribution that's doing it.


PairTheBoard

The two big differences that I can think of is that the hot water would have less dissolved gas due to degassing at high temperature and that there would be less hot water due to evaporation when the water was hot.

This is a tough thing to deal with because there are so many variables. One big question is the temperature of the freezer. Dissolved gas lowers the freezing point of water. If the freezer is only slightly below 0 C, you could actually have a case where the hot water freezes and the cold water never freezes because the dissolved gas lowers its freezing point below the temperature of the freezer.

Water does expand when it freezes. It reaches its highest density at 4 C and the density decreases from there until it freezes. Hockey is one issue, but more importantly if this didn't happen ice would sink when it formed on water leaving the surface exposed to form more ice. Lakes in cooler climates would form a lot of ice in the winter. Then in the summer the dense ice would remain at the bottom while the upper surface melted. It would be very likely to last through the summer. Fairly deep lakes could essentially freeze solid in a few years. It would be a big inhibiter to any aquatic life.

That said, the energetics in your bold comment are all wrong. Melting ice takes a lot of energy. That is why a piece of ice at 0C will cool a drink far more than putting in an equal amount of 0C water. Evaporating ice will take far more energy than evaporating water.

This was a joke, right?

Exactly, and now for his punishment he is "ordered" to calculate the period of a sliding ball in a parabolic mirror (y=ax^2) as function of the starting height under gravity g and realize if what he said is true or not even in the limited sense.

It freezes faster because of gravity.

I was looking up the solubility of oxygen and nitrogen in water to try to rough out some numbers to see if gas induced freezing point depression was workable as an explanation of this effect. As I did, I stumbled onto this reference:

http://arxiv.org/pdf/physics/0604224.pdf

This looks very plausible to me. Particularly if you define freezing to be the moment when the water is totally frozen rather than when the first ice forms. I had forgotten about the effect that the water excludes particulates as it freezes. Thus the surface skims over and the gas solutes are trapped in the body of the cube. As it freezes, more pure water is removed and the solute concentration increases, increasing the freezing point depression. The result is that the water has to be cooled well below 0 C before it will freeze if it starts with a significant concentration of impurities.

This is where the temperature of the freezer comes into play. The rate of heat removal is (to a first order approximation) proportional to the temperature difference Tw - Tf where Tw is the temperature at which the water is freezing and Tf is the ambient temperature of the freezer. Taking an extreme example, if the freezer were at -4C and the freezing point depression were 2C due to gases, it would take twice as long for the last increment of heat to be removed from the cold water sample as from the hot which would have no solute induced freezing point depression. If the temperature of the freezer were very cold, say -79C (dry ice) then the 2C freezing point depression would only slow heat removal by 2.5%. That would not be enough to overcome the high temperture starting point.

The paper also notes that the effect was not observed in degassed water samples. That almost confirms that the author has the right answer.

I think this is a pretty solid answer. The problem is not nailed because of all the variables around other possible solutes due to impure water, the effect of the freezer temperature, the concentration of other impurities and the definition of when a sample has frozen. But I would say that there is no mystery here.

Bottom line we need to know the exact conditions of any experiments they have done including extensive composition of the water used and its "history".

That effect seems totally trivial to isolate and define. I mean all it takes is 4-5 different experiments and you already know what is causing it or if its really very intriguing (probably not).

So then i ask why on earth havent they done these simple experiments to better define the phenomenon and come up and ask the right questions instead of some generic ones.

As i have said before and RLK also illustrated nicely the specific details of the water used and the cooler will tell you a lot. We didnt even define what freezing is or how close the masses of the 2 water vessels are.

At the very least what they can do is try to replicate the effect with water that had been repeatedly boiled many times to remove as much gases as possible. If they cant reproduce it with such water (say hours after being boiled and cooled) then you have your key cause right there.

Furthermore we need to know how water absorbs gases and what boiling does to them and how they affect the freezing process (hence the suggestion to redo the experiment with boiled water after many hours).

Eg read;

http://www.engineeringtoolbox.com/ai...ter-d_639.html


The relative concentration of impurities in water also affects the results. Maybe not by a lot but it depends on the details. When you boil you reduce the amount of gases dissolved and also lose some water to vapor. This will modify freezing point slightly if you are not careful (eg start with 1 lit water in 2 vessels and boil one losing gases and some vapor meaning the concentration of both gases and mineral in the resulting boiled water is now different eg one is 1lt the other is now 0.9 without dissolved gases. Also the fraction of boundary surface of water that comes in contact to container walls or air is different in the 2 vessels now affecting cooling process).