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, firstname.lastname@example.org
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: http://www.gunn.palo-alto.ca.us/k6science/water/w_q_a/expand.html A jumping-off point to other Ask-An-Expert services: http://www.askanexpert.com/askanexpert/ -Doctor Ken, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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