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One of the wonderful properties of water (as my high school biology teacher would say) is that in its solid form, it is lighter than its liquid form. This means that when temperatures drop below 0 degrees Celsius, the top layer of water on, say, a lake freezes first. This works out pretty well for any fish or other aquatic creatures living underneath, because the lower layers of water will not freeze.

I know that this is an exception to the general rule of solid and liquid states: A given substance, when transformed into its solid state, will generally sink in a container of its liquid state. My question is this: What other substances are exceptions to this rule (if any?). What features do they share with water that are responsible for this?

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  • $\begingroup$ Your second question is probably not physics-related anymore. $\endgroup$ – Phonon Aug 19 '14 at 18:19
  • $\begingroup$ I know; I felt it merited mention, though. $\endgroup$ – HDE 226868 Aug 19 '14 at 18:20
  • $\begingroup$ Just a word of advice before mods flag your post: Your first question doesn't ask for any physics, because you're not asking about the "why", instead it goes rather in the direction of material sciences or chemistry. I would just remove the second question, or make it into a statement. $\endgroup$ – Phonon Aug 19 '14 at 18:38
  • $\begingroup$ Thanks for the advice. I decided to delete the last bit because it really doesn't add anything. $\endgroup$ – HDE 226868 Aug 19 '14 at 18:42
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Start of an answer... hoping someone else will edit / comment / improve.

The reason that water expands on freezing is that the crystalline state has a specific orientation of the molecules (through hydrogen bonds) that leaves a lot of space between them. So where most of the time the liquid is a "messy form of the solid" and therefore takes more space, for water the crystal lattice is wide open. From http://johncarlosbaez.wordpress.com/2012/04/15/ice/

enter image description here

The key property here is that the molecule is polar, so there is a definite charge distribution on the molecule; this in turn favors a particular relative orientation of the molecules; and finally the way the molecule is angled ensures that a specific (energetically favorable) orientation leaves a relatively large amount of open space - it forms a tetrahedral lattice.

According to http://www.sciences360.com/index.php/substances-that-expand-when-they-freeze-24357/, other substances that form tetrahedral lattices (presumably because of the way their electrons are arranged) include silicon, bismuth, antimony and gallium.

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    $\begingroup$ You can add germanium as well. While both Si and Ge are diamond-cubic semiconducting solids, they are 6- to 12-fold coordinated metallic liquids. The density difference for Si is close to 10%. Bismuth and antimony are rhombahedral 'metals', and metallic liquids. At least for Bi, the density difference is ~3% (often used in braze and weld materials to prevent compressive strain after solidification). $\endgroup$ – Jon Custer Aug 19 '14 at 23:27
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    $\begingroup$ +1. There's an error in the linked article at sciences360.com: It should be noted that liquid water does NOT expand as it gets colder. water at 41 degrees F is DENSER than water at 42 degrees F. Water DOES expand as it gets colder, once the temperature drops below 3.98 °C (39.16 °F). Water at 38 °F is less dense than water at 39 °F. Fresh water has a density maximum at 3.98 °C (39.16 °F). $\endgroup$ – David Hammen Sep 30 '14 at 20:13

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