Why does hot water sometimes freeze faster than cold water?

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Composition and transformation of substance.

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Why does hot water sometimes freeze faster than cold water?

#1  Postby trubble76 » Jun 26, 2012 3:14 pm

We know that Aristotle said: "All paid jobs absorb and degrade the mind." We can be pretty sure he said: "One swallow does not make a summer." But we can hazard a guess that at some stage he would also have declared angrily: "This bloody ice thing is doing my head in."

The great Athenian polymath couldn't solve the mystery of why hot water sometimes freezes faster than cold water. He must have been annoyed to be defeated by what appears to be such a simple question.

But here we are, more than two thousand years later and we still don't know the answer to what has become known as the Mpemba effect. The moon has been walked upon, Mars has been explored, hearts transplanted – but that old chestnut of ice and hot water still baffles humankind.

The Royal Society of Chemistry has decided enough is enough. In an attempt to nail the matter once and for all, we're asking the public to come up with a convincing explanation of a phenomenon that defeated Aristotle, Francis Bacon and René Descartes. To win a £1,000 prize they will need to make a convincing case and employ some creative thinking.


From here.

I've never heard of this effect before, I will have to do some reading. Any thoughts from you clever people?
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Re: Why does hot water sometimes freeze faster than cold water?

#2  Postby BlackBart » Jun 26, 2012 3:42 pm

A complete stab in the dark...

Maybe the warm water has more impurities suspended due to convection in it that ice crystal can form on, whereas in cooler water the impurities would tend sink to the bottom?
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Re: Why does hot water sometimes freeze faster than cold water?

#3  Postby mattthomas » Jun 26, 2012 3:43 pm

I've heard of the effect but never experimented to check it out. I'm going to throw my uneducated mind into the fray and postulate that in heated water the atoms are more loosely spaced (less bonded hydrogen/oxygen atoms) and as in order to freeze, water assumes a uniform structure (hence why it expands when frozen) that the atoms may be more "free" to assume this structure under rapid cooling than the more closer bound atoms in cool water.
Last edited by mattthomas on Jun 26, 2012 3:44 pm, edited 1 time in total.
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Re: Why does hot water sometimes freeze faster than cold water?

#4  Postby trubble76 » Jun 26, 2012 3:43 pm

BlackBart wrote:A complete stab in the dark...

Maybe the warm water has more impurities suspended due to convection in it that ice crystal can form on, whereas in cooler water the impurities would tend sink to the bottom?


Interesting, that could be quite easy to test I reckon. Fancy a grand?
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Re: Why does hot water sometimes freeze faster than cold water?

#5  Postby The_Metatron » Jun 26, 2012 3:44 pm

Less dissolved gas in hot water.
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Re: Why does hot water sometimes freeze faster than cold water?

#6  Postby trubble76 » Jun 26, 2012 3:46 pm

Three excellent suggestions already! I knew you guys wouldn't disappoint. :popcorn:
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Re: Why does hot water sometimes freeze faster than cold water?

#7  Postby mattthomas » Jun 26, 2012 3:50 pm

You're just going to nick them all and claim the grand aren't you?
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Re: Why does hot water sometimes freeze faster than cold water?

#8  Postby trubble76 » Jun 26, 2012 3:52 pm

mattwilson wrote:You're just going to nick them all and claim the grand aren't you?


Yoink! :lol:
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Re: Why does hot water sometimes freeze faster than cold water?

#9  Postby BlackBart » Jun 26, 2012 3:54 pm

trubble76 wrote:
BlackBart wrote:A complete stab in the dark...

Maybe the warm water has more impurities suspended due to convection in it that ice crystal can form on, whereas in cooler water the impurities would tend sink to the bottom?


Interesting, that could be quite easy to test I reckon. Fancy a grand?


I'm in the kitchen as we speak...does instant coffee count as impurities? :think:
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Re: Why does hot water sometimes freeze faster than cold water?

#10  Postby trubble76 » Jun 26, 2012 3:55 pm

BlackBart wrote:
trubble76 wrote:
BlackBart wrote:A complete stab in the dark...

Maybe the warm water has more impurities suspended due to convection in it that ice crystal can form on, whereas in cooler water the impurities would tend sink to the bottom?


Interesting, that could be quite easy to test I reckon. Fancy a grand?


I'm in the kitchen as we speak...does instant coffee count as impurities? :think:

As a tea drinker, I'd say yes, certainly. :yuk:

Edit to add: On a more serious note, would adding salt not count as impurities? But salt water doesn't freeze as fast so perhaps we can discount that hypothesis.
Last edited by trubble76 on Jun 26, 2012 3:56 pm, edited 1 time in total.
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Re: Why does hot water sometimes freeze faster than cold water?

#11  Postby Microfarad » Jun 26, 2012 3:55 pm

The_Metatron wrote:Less dissolved gas in hot water.

Possible.
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Re: Why does hot water sometimes freeze faster than cold water?

#12  Postby Ironclad » Jun 26, 2012 3:55 pm

It's the gas and the mineral thing. Cheque please.
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Re: Why does hot water sometimes freeze faster than cold water?

#13  Postby Ironclad » Jun 26, 2012 3:57 pm

"If there was no such thing as science, you'd be right " - Sean Lock

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Re: Why does hot water sometimes freeze faster than cold water?

#14  Postby Microfarad » Jun 26, 2012 3:59 pm

Ironclad wrote:http://library.thinkquest.org/C008537/cool/freeze/freeze.html

Thank you!
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Re: Why does hot water sometimes freeze faster than cold water?

#15  Postby mattthomas » Jun 26, 2012 4:01 pm

trubble76 wrote:
BlackBart wrote:
trubble76 wrote:
BlackBart wrote:A complete stab in the dark...

Maybe the warm water has more impurities suspended due to convection in it that ice crystal can form on, whereas in cooler water the impurities would tend sink to the bottom?


Interesting, that could be quite easy to test I reckon. Fancy a grand?


I'm in the kitchen as we speak...does instant coffee count as impurities? :think:

As a tea drinker, I'd say yes, certainly. :yuk:

Edit to add: On a more serious note, would adding salt not count as impurities? But salt water doesn't freeze as fast so perhaps we can discount that hypothesis.

Try freezing hot salt water... salt water is typically cold.
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Re: Why does hot water sometimes freeze faster than cold water?

#16  Postby byofrcs » Jun 26, 2012 4:07 pm

Yes I saw someone who made icecream discover this effect.....

http://en.wikipedia.org/wiki/Mpemba_effect
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Re: Why does hot water sometimes freeze faster than cold water?

#17  Postby Curli » Jun 26, 2012 4:25 pm

http://math.ucr.edu/home/baez/physics/G ... water.html

A good overview^

Cold water should all things being equal if you place it outside in sub zero weather freeze faster than hot water, reason being obviously that cold water has less energy to lose before it gets to its freezing point but as the above article says all things are not always equal.

I would of said something like wont putting a hot thing in the freezer cause the freezer to become cooler, so that as it cools it would be at a lower temperature than before had the water been cool enough not to set the freezer off, but since in the experiment the hot and cool are in the same freezer, that clearly is not the case. Having read the links though the reasons given do make sense, it is not magic. :)
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Re: Why does hot water sometimes freeze faster than cold water?

#18  Postby chairman bill » Jun 26, 2012 5:49 pm

Hot water cools more quickly because of the added cooling effect of evaporation.
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Re: Why does hot water sometimes freeze faster than cold water?

#19  Postby virphen » Jun 26, 2012 6:16 pm

I am showing my ignorance I know, but I always thought it was to do with the mathematics of rates of change. I was surprised to see it's a mystery, which makes me think that's just another piece of bullshit which pervades life that I've accepted uncritically.

Shame on me.
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Re: Why does hot water sometimes freeze faster than cold water?

#20  Postby I'm With Stupid » Jun 26, 2012 6:24 pm

£1000? Don't you usually get a Nobel Prize and over a million for solving the deep mysteries of science?
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