In general, when a pure substance or mixture goes from liquid to solid, the molecules in it get closer resulting in a higher density.

If this is so why does ice float in water?

The reason does have to do with the hydrogen bonds that water has. They allow water to form 4 tetrahedral bonds (similar to the structure of diamond) with each other into a big network in its crystalline form. Actually, water forms about 19 different forms but the one we see is the most stable form in normal conditions. (Well, my college chem professor calls himself Captain Hydro and he's been studying water for 10-15 years and he talks about it everyday.)

Because of the size difference of the hydrogen and oxygen and the structure of the network, there are many holes in it. While ice melts, the structure collapses and these holes get filled up until about 4 degrees. After the holes are filled up, the water starts to expand as you increase the temperature.

This is one property of water that makes it very important to life, especially to fish in cold climates. A layer of ice freezes forms over the body of water when it's cold which forms a layer of insulation for the fish so that they do not freeze to death.

Posted by np_rt on 2003-03-24 08:19:34