
Mpemba effect: warmer water can freeze faster than colder water - akircher
https://en.wikipedia.org/wiki/Mpemba_effect
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sandworm101
A slightly different version of the effect well-known to all Canadians:

[https://www.youtube.com/watch?v=pvgefs7J1rY](https://www.youtube.com/watch?v=pvgefs7J1rY)

[https://www.youtube.com/watch?v=ZGjwe-
BCfms](https://www.youtube.com/watch?v=ZGjwe-BCfms)

To get the water to freeze into snow mid-air, you have to start with boiling
water. Cold water won't freeze in time. I've even heard of research involving
using boiling water in snow-making machines.

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sndean
> using boiling water in snow-making machines

The seems like a related effect:
[https://www.youtube.com/watch?v=IQ3kQPWlU-o](https://www.youtube.com/watch?v=IQ3kQPWlU-o)

So evaporating water cools the surrounding air?

~~~
sandworm101
Yes. To transition from liquid to gas the water must absorb heat. That heat
must come from the air, cooling it.

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nkurz
I've always disliked the explanations I've seen for the Mpemba effect. They
seem to be based on poorly defined terms, odd assumptions, and uncontrolled
variables. The (poorly) linked 2010 paper by Brownridge is the first
description that I'm inclined to trust:
[https://www.binghamton.edu/physics/docs/Preprint%20and%20Sup...](https://www.binghamton.edu/physics/docs/Preprint%20and%20Supplemental%209%20Mar%2010.pdf)

Impressively, he describes a controlled duplicable setup in which he's able to
reliably freeze (using an exact definition) identical quantities of water in
identical containers under identical conditions such that the hot water
freezes before the cold water. Incredibly, he was sometimes even able to
observe this when starting with 100C water vs 0C water!

As you'd guess, there is a little bit of a trick to it, but it's one that not
nearly as obnoxious as some of the earlier explanations: no evaporation
resulting in a smaller volume or remaining water, no spontaneous ice spikes
with greater surface area, and no differential melting of the thick layer of
frost on the freezer shelf required.

Rather than giving a spoiler, I'll let the paper describe the exact technique
he used. OK, a hint from the paper to encourage you to actually read it: cold
water will indeed always reach 0C before identically handled hot water, but
contrary to popular belief reaching 0C is merely a requirement for freezing
rather than a guarantee.

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PepeGomez
I think it's because there is more gas in cold water. This results in ice that
has more bubbles in it and conducts heat more poorly, and prevents the latent
heat from being removed from the yet unfrozen parts of the water.

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ohwaitnvm
I googled around and found this paper to be far more compelling than any of
Wikipedia's vague explanations:
[http://arxiv.org/abs/1310.6514](http://arxiv.org/abs/1310.6514)

~~~
Aelinsaar
Driving home once again, what an interesting and unusual molecule our
ubiquitous friend 'water' really is. Polar, bent, and funky; expanding when
frozen, incompressible as a liquid.

~~~
cordite
Is it truly incompressible, or just enough for practical uses?

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angersock
It's only like ten times more compressible than diamond, if I remember my
schooling correctly.

In liquid water, mechanical engineers are usually free to assume
incompressability. This makes solving the math a lot easier, though absolutely
cannot be done for gases.

~~~
SilasX
Well, even for gasses, your results are accurate enough as long as the Mach
number is below 0.3:
[http://encyclopedia2.thefreedictionary.com/incompressible+fl...](http://encyclopedia2.thefreedictionary.com/incompressible+flow)

~~~
calgaryeng
This is not done in practice. Be careful getting engineering advice from the
Free Dictionary.

~~~
SilasX
It reflected what I was taught in fluid/gas dynamics courses for my degree,
and you'll see the same point made in various other places.

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gregw134
I'm putting hot and room temperature water in my freezer...no way this will
work.

~~~
gregw134
Update: didn't work

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tiplus
Erasto Mpemba boiled milk mixed with sugar to make ice cream. The original
experiment is much more rewarding than simply putting water into the fridge.
;-)

~~~
gregw134
Good point...I should try again when I'm hungry :)

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DanBC
The Royal Society for Chemistry ran a competition to come up with an
explanation: [http://www.rsc.org/learn-
chemistry/resource/res00001018/the-...](http://www.rsc.org/learn-
chemistry/resource/res00001018/the-mpemba-effect?cmpid=CMP00007615)

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chrisBob
Seriously you should use warm water to fill your if cube trays. Even if it
doesn't freeze faster you get better ice cubes from having less dissolved gas.

~~~
gboss
As long as it's not hot/warm water from the faucet. Many water heaters can
leach metals into the water and while safe for showering you shouldn't drink
it.
[http://mobile.nytimes.com/2008/01/29/health/29real.html?refe...](http://mobile.nytimes.com/2008/01/29/health/29real.html?referer=)

~~~
aaron695
More importantly they taste like crap.

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mark-r
They mention the difficulty of determining when the water is frozen, since
water at zero degrees C can be either liquid or solid. It seems easy to solve
by checking for an increase in volume or pressure, since we know that ice
expands into more volume than liquid water.

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taejo
If you track temperature _over time_ you can tell when freezing starts and
ends: when freezing starts, the temperature stops dropping, and it starts
dropping again when freezing is complete.

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basicplus2
nothing misterious really.. it only works where hot water turns more or less
to steam ie very high surface area and thus can cool faster thus overtake
cooler water droplets that have significantly less surface area.

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arjuntuli
After reading this article, I did some research on this topic and compiled
some of the best resources I found here - [http://knowledgemaps.org/learning-
journey/1118/understanding...](http://knowledgemaps.org/learning-
journey/1118/understanding-the-mpemba-effect)

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junto
Reminds me of an argument my brother and I used to have.

You pour two cups of tea. You add an exact amount of cold milk to one but not
the other then wait five minutes. Then you pour the same amount of milk in the
second cup. You stir both equally.

Which is colder?

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BrandoElFollito
This one is simple. The more difference in temperature you have, the faster
the liquid is cooled down. This means that you should first let the tea cool
by itself for 5 minutes, then to cool it with milk to get the colder version.

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adontz
This was presented as problem #15 (Cooling water) at IYPT 2000

As far as I remember, nobody confirmed effect experimentally at that time.

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tropo
I think I know why.

Good temperatures for showing the effect are 5 and 35 C.

5 C is near 4 C, the point at which water has maximum density.

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sogen
Best explanation I saw was from Beakman's World, a 90's Tv show.

Sorry can't find a YouTube video.

