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Expanding Water

Date: 03/13/97 at 15:17:19
From: Robin Pogrebin
Subject: Homework help

I'm a reporter for the New York Times writing a story about electronic
homework help. Could you please answer the following question: Why 
does water expand when it freezes when other materials contract? 

Many thanks,

Date: 03/13/97 at 16:15:52
From: Doctor Ken
Subject: Re: question-urgent 

Hi there -

To answer this question I'm going to draw upon my Minnesota roots - 
we've got plenty of freezing expanding water around to contemplate!  

As I learned in my science classes, water is a polar molecule. This 
means that if you get yourself an H2O water molecule and hold it up, 
one end of it will carry a negative electrical charge, and the other 
end will carry a positive charge.

This small fact has incredibly far-reaching consequences for water's 
behavior. For one thing, it makes water comparatively sticky, since 
the negative ends of one molecule will be attracted to the positive 
ends of other molecules. If you compare water to other molecules of 
about the same size, you'll see that (if the other molecule is even a 
liquid at the same temperature) water's drops are usually bigger, and 
water tends to be more cohesive than the other guys. Chemists call 
this stickiness hydrogen bonding - the force drawing one molecule to 
another is called a hydrogen bond.  

Now, as far as I know (and I could be wrong here) the simple water 
molecule still isn't all that well understood. But we do know that as 
the temperature of water changes, these hydrogen bonds contract and 
relax, and thus the distance between water molecules changes. As 
water cools from 100 degrees to 4 degrees Celsius, the hydrogen bonds 
get stronger and water contracts. At 4 degrees water is at its most 
dense. As water cools from 4 degrees down into freezing, the hydrogen 
bonds relax and the water expands. This is why frozen water is less 
dense than liquid water.

Incidentally, the fact that water is at its most dense has deep 
consequences in lake ecosystems. If the lake is big enough, the 
temperature at the bottom may be 4 degrees the whole year long.  
There's also a place in the lake, called the thermocline, where the 
temperature makes a sudden jump from warm to cold. This thermocline 
moves up and down in the lake with the seasons, and the motion of the 
thermocline helps give life to the ecosystem.

Since I'm a mathematician and not a chemist, I'm afraid to say much 
more than that for fear of being wrong!  But here are some resources I 
and my team found on the web that will help you if you want to pursue 
this further.

Several answers to the question by quite qualified people:   

A jumping-off point to other Ask-An-Expert services:   

-Doctor Ken,  The Math Forum
 Check out our web site!   
Associated Topics:
High School Physics/Chemistry

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