|Gail - |
Actually, most liquids do /not/ expand when they freeze. In fact, most liquids lose volume as they freeze. This is because when you freeze a liquid, its molecules slow down. The more they slow down, the tighter the bonds are between them. And the tighter the bonds are between them, the closer together they become, usually. When the individual molecules become closer together, the substance will take up less space.
Water is one of the rare liquids that forms a crystalline solid that takes up more space than the liquid itself does. The reason for this is purely incidental (although important for many things, as you mentioned). Because of the shape of the water molecule and the angles that it forms when it bonds, the solid form of water actually ends up taking up more space than liquid. Like I said, this is /very/ unusual. There are a few other substances that behave the same way, but not many at all.
For more information, check out the answer to the question "Freezing Water" in the Solids, Liquids, and Gases - Liquids section of our answers list.
(republished on 07/25/06)
(published on 12/14/07)