| MadSci Network: Physics |
Dear Glen,
The process of freezing (or solidification) is usually accompanied by an increase in molecular order. As the temperature gets lower and lower, intermolecular forces begin to establish some long-range order. This is not possible at high temperature because of the statistical nature of thermal movement.
In most cases this increased order means that molecules are more closely ``packed'' than before, hence the reduced volume. But, as you may already imagine, the exact way in which this packing occurs depends strongly on the molecular structure. The ``way of packing'' is called crystal structure in cases where a crystal is formed. The crystal structure of sodium chloride is different from that of diamond, for example. Crystals all have long-range order, as I mentioned above, but the actual molecular density in a crystal is determined by the molecules' properties.
Water molecules in particular (and some others, too) have the special property of an electric dipole moment, which means that the ``electron clouds'' from the two H atoms are ``pulled over'' to the O atom (chemists attribute this to the high electronegativity of the oxygen atom). This leads to a charge imbalance: The positive ``centre of charge'' does not coincide with the negative one, and from some distance the whole thing looks like two opposite charges which are a small distance apart. This is called an electric dipole.
This is one of the reasons why the freezing of water is a complicated process which can uccur in several different ways, depending on external conditions. Actually there is not just one kind of ice but several so-called modifications which are numbered ``ice I'' up to ``ice VII'' (if I remember correctly). The special thing about molecules like water is now that the crystal structures that emerge take up more space than the liquid form previously.
Hope that helps,
Georg.