As you probably already know, ordinary matter is made of atoms and molecules. Water is made of molecules that are composed of one oxygen atom and two hydrogen atoms, and the molecules have a bent shape, kind of like a V (the oxygen atom is at the point of the V, and the hydrogen atoms are on the "legs" of the V). If
you examine a water molecule, you will find that it has a partial plus
charge where the hydrogen atoms are and a partial negative charge where
the oxygen atom is. The partial plus charge of the hydrogen attracts
the partial negative charge of the oxygen, and this attraction
(hydrogen bonding) makes the water molecules stick together. They stick together best when they form a regular pattern. That regular pattern is a rigid crystal: ice.
So why isn't water always frozen into ice? If you could look at the individual molecules in any piece of matter, you would see that they sort of jiggle around, that is, they have kinetic energy. Molecules have less kinetic energy at lower temperatures and more kinetic energy at higher temperatures. At high enough temperatures, the molecules shake so much that they break away from the regular frozen pattern and melt. When they get even hotter, they can break away from sticking to each other at all and turn to a gas.
You may wonder why the change from liquid to solid is all-or -nothing. Why donít the molecules just stick more and more as they get colder? We can sort of explain. In the solid, the molecules line up in a regular pattern, where each molecule just fits in a framework provided by its neighbors. A molecule canít do that unless its neighbors are lined up properly. Once the neighbors are lined up, a molecule can lower its energy a lot by joining the pattern. So it becomes all-or-nothing.
Some materials, like window glass, are solid-feeling but donít actually consist of regular patterns of lined up molecules. If my explanation is right, you would then not expect them to freeze abruptly but rather more gradually. Thatís just what happens.
Thanks for your question!
(republished on 07/25/06)