How can the Anomalous Expansion of Water from 4$^\circ$C to 0$^\circ$C be explained with reference to subatomic particles?


I don't think there's a good general explanation of this; the best I can do is give a few hand waving arguments.

If you look at hydrogen sulphide, which the analogue of water moving one row down in the periodic table, you'll find it does shrink when it freezes, just like most other liquids. So the difference between the H$_2$S and H$_2$O molecules must be responsible for the anomalous behaviour of water.

The most obvious difference between the two molecules is the the H-O bond is highly polar and has a strong electric dipole associated with it. This means it interacts strongly with other H-O bonds; the interaction is known as hydrogen bonding. Hydrogen bonds are highly directional, as you'd expect for an electric dipole, so the water molecules can be fitted together in ice any old how. They adopt well defined positions relative to each other, and the directionality of the bonds forces the water molecules into a relatively low density arrangement.

It's interesting to note that the high pressure forms of ice are generally denser than water. Presumably at high pressure the reduction in energy by the denser arrangement outweighs the reduction in the strength of the hydrogen bonds.

  • $\begingroup$ Has it nothing to do with the closed loop structure of attachment of the water molecules? $\endgroup$ – Graviton May 31 '12 at 15:43
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    $\begingroup$ Do you mean the way the water molecules in ice form hexagons? If so then yes, the hexagonal structure is a consequence of the directional hydrogen bonds. $\endgroup$ – John Rennie May 31 '12 at 16:37
  • $\begingroup$ Yes, that's what I mean. So do the hexagons become immobile for some reason, due to which further compression ceases to occur? $\endgroup$ – Graviton May 31 '12 at 16:40
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    $\begingroup$ Above 4°C the hydrogen bonds are being continually broken and remade by the motion of the water molecules so there are no hexagonal structures. Below 4°C the molecular motion is too slow to completely disrupt the hydrogen bonds so you start getting temporary structures forming, including the hexagonal rings. If you supercool the water it becomes more structured and less dense, though still not as structured as ice. The solid ice grows by nucleatuon of an ice crystal and attachment of water molecules to the surface of the solid, so hexagonal rings in the liquid don't correspond to the ... $\endgroup$ – John Rennie May 31 '12 at 16:47
  • $\begingroup$ hexagonal rings in the solid. I mean a ring of six water molecules doesn't all freeze at once. $\endgroup$ – John Rennie May 31 '12 at 16:48

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