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There is this puzzling thing that is called Mpemba effect: paradoxically, warm (35°C) water freezes faster than cold (5°C) water. As a physisist, I've been asked about it several times already. And I have no definite answer to that, apart from the standard: "there are many things that can influence it".

So, does anyone knows about the status or progress on that effect? Any recent reviews, publications or other references?

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Relevant: hermes2012.org/ice –  Emilio Pisanty Jul 28 '12 at 0:21
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Thanks for the link. The winner's conclusion is that the effect in largely due to supercooling and convection (see rsc.org/mpemba-competition/mpemba-winner.asp), quite in contradiction with the answer by John Rennie and Ron Maimon. Does this mean that the investigations have not yet given a good commonly accepted answer? –  UwF Jan 3 at 21:13
    
Related mathoverflow question: mathoverflow.net/q/153669/13917 –  Qmechanic Jan 6 at 0:55

3 Answers 3

One boring Monday morning in the lab a group of us did the experiment, and to our surprise we found that the hot water (in sealed containers) did freeze faster.

On closer examination we discovered that the shelves in our freezer were covered in frost, like I imagine most freezers, and the hot water was melting the frost and creating a good thermal contact between the beaker of water and the shelf. That turned out to be why the hot water froze faster. When we thoroughly cleaned the freezer shelf the effect went away and the hot water took longer to freeze.

I think the rumours about hot water freezing faster illustrate the dangers of improperly controlled experiments. As Ron mentions, evaporation could also be a factor and it would be easy for a home experimenter to get the wrong conclusion. Add to that the fact we'd secretly all be delighted if we could prove hot water really does freeze faster, and you can see how the rumour has spread.

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+1, that's an effect I never considered! I still think it's not nice to name a bogus effect after a student, it associates all sorts of mystical ideas of water-memory and so on with some guy who never made any such claims--- who just noticed that it freezes faster in an uncontrolled way. –  Ron Maimon Jul 28 '12 at 7:15
    
"...home experimenter to get the wrong conclusion" Meahh, two weeks ago, I did the experiment twice with some of my own protection gears. But well, it turned out that the effect didn't appear. Instead, I got the usual cooling curves both the times.Trial 1 (somewhat wrong-ey), Trial 2 (quite righty) :D –  Waffle's Crazy Peanut Dec 13 '13 at 15:22
    
have a look at this medium.com/the-physics-arxiv-blog/d8a2f611e853 the paper arxiv.org/abs/1310.6514 –  anna v May 4 at 15:41
    
@John What do you say about this explanation? –  Yashbhatt Jun 12 at 13:25
    
Cool 2 containers one with 75 F temp and one with 150 F in the same freezer at the same time. Both of them will in relatively insignificant time melt the frost between the container base and the freezer metal. So the frost effect can only be at play in a horribly frosted freezer where one container will have enough heat to melt surrounding ice and end up touching the freezer metal bottom while the other may not. So @John your theory can't be used to deny the effect that's been observed by many. Your starting with a sloppy freezer makes me want to question the reliability of your experiments. –  LMSingh Sep 27 at 20:37

The explanation is that hot water evaporates, leaving less water to freeze (but see John Rennie's answer for a contact effect which is probably the biggest factor for most of the reports of the effect. Aside from John's effect...) this is the only significant difference, there is no other, despite what you sometimes read. The evaporation effect is not even that big, if you add 10 degrees of temperature to the water, the difference in evaporation is negligible and the hot water will freeze by an amount of time which is longer by exactly the time it takes to cool down the extra 10 degrees.

All experiments that claim that a closed container of hot water freezes faster are unreliable (but see John Rennie's answer). But nobody freezes in closed containers for obvious reasons. If you freeze trays covered with plastic-wrap, hot water freezes slower. There is no "Mpemba effect", and I feel it is not nice to attach such a thing to an obscure student who was reporting something true from experience.

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Have you yourself done any experiments on this? –  UwF Jan 3 at 21:15

About 7 months ago I was interested in this topic and discovered that this paper sheds new light on Mpemba's effect. It was then claimed to be "the ultimate answer". Check it out: http://arxiv.org/abs/1310.6514

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