|MadSci Network: Physics|
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,
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