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

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

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

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.

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

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.)

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.

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.

Well…what I’m reading right now in Wikipedia is this:

And later:

So Wikipedia does not agree with Cecil.

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.

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.

This proves one thing.

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

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.

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?

[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! :eek:

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

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.

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.

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.