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Why does a substance expands upon freezing? What conditions necessitate this? For example, how does the slope of sublimation or fusion curve in a P-T diagram affect this?

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    $\begingroup$ Very few substances do this, and I have a vague memory suggesting that they all have oddball bond geometry. Can't provide a citation and may be way off base. $\endgroup$ – dmckee Nov 4 '14 at 20:00
  • $\begingroup$ All first order phase transitions must have a change of volume. It can either increase or decrease. Water is the everyday example of a substance that increases in volume upon freezing. Amongst the elements, at least Si, Ge, Sb, and Bi do as well. While Sb and Bi are rhombahedral, while Si and Ge are diamond cubic. $\endgroup$ – Jon Custer Nov 4 '14 at 20:14
  • $\begingroup$ Water is one of the few substances to expand upon freezing at $4^\circ C$ by about 9% since the formation of the hexagonal lattice which contains more space than the liquid form. Water is a peculiar substance with a lot of interesting properties due to it's molecular shape, composition, and tendency to form H-bonds. $\endgroup$ – user43617 Nov 4 '14 at 20:15
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I agree that ice has more free space in its solid form. All the bonds, polar bonds, and nuclei form a lattice. Most frozen substances contain less free space than the liquid phase. Well, I think the better way to think of it is as solid water has a high amount of free space compared to other similar solids. Entropy would drive the electrons into the free spaces, since there are probably low energy states for e pairs in the open spaces. It must be that the drive for the lattice structure is high. So ice maybe in a fine balancing act. Change, the energetics of the lattice structure a bit and the hole thing could collapse. The interested reader can look up other forms of ice that form under various pressures. These may be the above "hypothetical" collapsed forms. I have looked up some of these other forms. Ice III is the one the forms most readily. It has something like 1.1 kg /L for its density. Something you would expect.

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The reason for this, simply put, is because of the structure of the lattice formed by the molecules upon freezing. For substances that exhibit this expansion upon freezing, the lattice structure happens to take up more space than when it is a liquid. For example, water (due to the formation/angle of the hydrogen bonds with the oxygen atom) has a hexagonal lattice structure when it's frozen, which takes up much more space than its liquid water molecule free-roaming state.

Check this link out.

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  • $\begingroup$ This is the prototypical answer. Is it possible to add a physicist's perspective or more quantitative information? $\endgroup$ – user43617 Nov 4 '14 at 20:22
  • $\begingroup$ @Jun-GooKwak, what exactly is a physicists perpsective? mathematical-physicists perspectve, physical inuition, both, sth else? $\endgroup$ – Nikos M. Nov 5 '14 at 0:05

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