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Hot water can cool faster than cold

by pitot-tube
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pitot-tube
#1
Jan23-08, 07:12 AM
P: 80
Experiments have been done in which one volume of water that is warmer than an identical volume of water freezes faster than the cooler volume.Since ice is an ordered structure compared to liquid water does this mean that the walls of the vessel containing the warmer water are able to interact with the water and order it quicker than the walls of a vessel containing colder water.Are the walls of the warmer vessel allowing some sort of adsorption process to occur more frequently than the walls of the cooler vessel?
Or is warm and more turbulent water able to shed heat faster?
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#2
Jan23-08, 07:25 AM
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Quote Quote by pitot-tube View Post
Experiments have been done in which one volume of water that is warmer than an identical volume of water freezes faster than the cooler volume.Since ice is an ordered structure compared to liquid water does this mean that the walls of the vessel containing the warmer water are able to interact with the water and order it quicker than the walls of a vessel containing colder water.Are the walls of the warmer vessel allowing some sort of adsorption process to occur more frequently than the walls of the cooler vessel?
Or is warm and more turbulent water able to shed heat faster?
It depends on the experimental conditions: if you use large aperture containers with not very much water inside and put them in the fridge, then the hotter water will vapourize more than the colder and condense into the fridge, so the container will soon have less water than the other and so it could freeze faster for just this reason.
pitot-tube
#3
Jan23-08, 12:56 PM
P: 80
The rate of freezing doesn't always depend on apertures of any kind.It is well documented that closed hot water pipes can freeze faster than closed cold water pipes.
I think that in some instances of this phenomenon the hotter water is heating the surrounding air, drawing in colder, denser air that then cools the hotter water faster.

chemisttree
#4
Jan23-08, 02:23 PM
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Hot water can cool faster than cold

I've heard of this and have an explanation that you can mull over. Hot water isn't the same as cold water. Cold water comes directly from the source of water while hot water stays in a relatively large, hot container. Minerals (especially carbonates) precipitate from hot water onto the interior of the hot water tank and the remaining water is relatively deficient in suspended solids as a result. This has unique consequences regarding the bursting of pipes.

The water in the cold water lines actually freezes slightly faster than the water in the hot water lines but this freezing doesn't occur at exactly 0C. Water begins to freeze at slightly less than 0C... this is referred to as supercooling. The higher levels of the suspended (not dissolved) solids in the cold water pipes initiate crystallization at a higher temperature (less supercooling). The freezing initiates at the wall (the coldest point in static water lines) and progresses inward. This occurs as fast as thermal diffusion will allow and the freezing walls actually heats the water in the center of the pipe to 0C. The freezing water expands and forces the interior water out the end of the pipe it came into... it's not a huge flow but it is greater than zero.

The situation in the hot water pipe is a little different. Without as much suspended solids to nucleate freezing water, the water supercools to a lower temperature. Now when the water eventually does freeze, it occurs much faster and it can prevent the water in the interior portion of the pipe from moving. Thus the trapped water freezes and exerts large expansive forces on the pipe causing it to burst.

Just a thought.
pitot-tube
#5
Jan24-08, 06:10 AM
P: 80
From what I can remember of this phenomenon people have done experiments where dissolved constituents were carefully controlled and everything was done to make the hot and cold sample of water identical and still they found that in some cases the hot water froze first.This is why I proposed some form of adsorption process on the walls of the container.Dissolved substances in water can lower its freezing point and perhaps in the hot water sample the substances are being adsorbed by or driven into the container wall lowering the concentration of dissolved substances and raising the freezing point.
There is also the idea that hot water in cold air sets up stress and strain in a metal pipe and that this is why a hot pipe bursts - a bit like metal fatigue!
I also think that in examples I've heard of where people pour hot water into a frozen bowl outdoors and it freezes much quicker than expected that what could be going on here is that there is lots of hot water vapour evaporating into the air which then cools rapidly and forms ice crystals that fall back into the bowl and provide nuclei for other crystals to form.
lisab
#6
Jan24-08, 07:46 AM
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I heard about this years ago - I think I was in high school - and didn't believe it. I tried it in my home freezer but didn't have the proper equipment to determine when the phase change was complete. What I did find was the hot ice tray melted into the ice on the shelf, resulting in better contact than the cold tray.

Can anyone find documentation of these experiments?
DaveC426913
#7
Jan24-08, 08:29 AM
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Also, as any aquarist knows, warmer water cannot hold as much oxygen as cold water.
kdv
#8
Jan24-08, 11:19 AM
P: 329
Quote Quote by lisab View Post
I heard about this years ago - I think I was in high school - and didn't believe it. I tried it in my home freezer but didn't have the proper equipment to determine when the phase change was complete. What I did find was the hot ice tray melted into the ice on the shelf, resulting in better contact than the cold tray.

Can anyone find documentation of these experiments?
Just in case it would help to do a search, this is called the "Mpemba effect"


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