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Published: 14 years ago
 
This is a reply to # 752,785

Off topic, but interesting


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Re: OMG! Hot water freezes faster than cold!
Posted by: Moreless (IP Logged)
Date: October 24, 2005 09:58AM

A much simpler explaination for why hot water starts to freeze before cold water is do to the fact that when water is heated one has what is called expansion or increase of space taken up by the water.

It is just an increase in the SURFACE AREA of the molecules which make up the body of water.

So when it is then subjected to cold for freezing you have a much larger surface area initially which will freeze faster than it's counterpart of cold water which has less SURFACE AREA.

Yet once the outer portions of the water are frozen , then it takes longer to freeze the inner portions, because of the change in conductivity by change of the simple particle size difference.

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Re: OMG! Hot water freezes faster than cold!
Posted by: Terry David Silvercloud (IP Logged)
Date: October 24, 2005 12:11PM

I had never heard of this before, and it does seem counter intuitive and against known physics, so I checked it out.

It isn't quite true, because it applies only to certain conditions. It is called the Mpemba effect after the person who did experiments to prove it.

What is wrong, or right, is that it depends upon the same volume of water at different temperatures. This immediately causes problems in the statement because the density of each is different, so you don't really have two identical "things" to begin with.

There is actually LESS water in the container of warmer, or hot water. ie, the experiment will ONLY work when the volumes are identical but with different temperatures, so if you stared with the same volumes at the same temperature, then raised on to a higher temperature, the colder of the two would freeze first... because it is already cooler.

It's about the same thing as trying to freeze a large piece of fruit, and a small piece of fruit starting from the same temperature. The smaller piece freezes first, which is logical in one's mind.
 

 
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