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In my vision it would seem quite logical that all materials expand when temperature rises. Because the particles get more energy and travel larger distances when moving. But apparently there are some materials that tend to shrink as temperature increases. Why? Which ones?

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  • $\begingroup$ Well, can you mention the ones you know? $\endgroup$ – rmhleo Jan 22 '16 at 13:47
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    $\begingroup$ should shrimp not be shrink? $\endgroup$ – Gobabis Jan 22 '16 at 13:50
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    $\begingroup$ @rmhleo, altough this is not the best example: water between 0 and 4 degrees celius has a negative expansion coefficient meaning its volume decreases. $\endgroup$ – privetDruzia Jan 22 '16 at 13:54
  • $\begingroup$ Well I think we have particular explanations for particular cases, but I don't think there is a general way of predicting which ones and why. It must be due to variations of intermolecular potentials with average motion of molecules The less kinetic energy of the molecule the more relevant the potentials are and more features can manifest, which are smeared out for haigh temperatures. $\endgroup$ – rmhleo Jan 22 '16 at 14:12
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Water has a very narrow range of temperatures over which it expands when cooling rather than contracting (IIRC +2 to 0 Celsius). This occurs due to the way the highly polar molecules "line up" with each other near the freezing point.

For a more interesting example, consider a number of rubber compounds which shrink as they warm up. In this case it's due to cross-linked molecules which paradoxically "relax" to a smaller configuration as they heat up. I'm not familiar with a good online set of diagrams for this; if I find one I'll post it.

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I imagine alot of negative expansion coefficients occour at phase transitions. Probably the temperature increase causes the material to overcome an activation energy and a change in lattice structure occours. In these cases one would not expect these processes to be reversible.

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  • $\begingroup$ and for a liquid ? $\endgroup$ – user46925 Jan 22 '16 at 14:46

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