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I have been told that water is the only substance that expands in both solid and gaseous states!

Is this true? If not, what other substances can do this?

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    $\begingroup$ I'm afraid water is not so unique. There are numerous examples of compounds with NTE, there is also Gough–Joule effect. Ga, Ge, Sb, Bi also expand upon crystallization from liquid phase. If you want, I will try so assemble these examples in an answer. $\endgroup$ – andselisk Jul 10 '17 at 18:42
  • $\begingroup$ I was wondering if it's the only naturally occurring substance that expands in both gaseous and solid form. My son says it's because of the polar nature of the molecules. Is this true, I would love to learn more! $\endgroup$ – Dick Patton Jul 10 '17 at 19:01
  • $\begingroup$ Ge, Ga Bi and Sb all appear to be in the "Poor Metals" group. Germanium is rather surprising as it was used in Semiconductors and I would think the expansion would be a problem with Junctions? Perhaps that's why they went with Silicon for Semiconductors. $\endgroup$ – Dick Patton Jul 10 '17 at 19:07
  • $\begingroup$ Actually, Si is also in this "anomaly" list. I guess I also forgot to mention Pu. Silicon substituted Germanium because it's literally dirt-cheap and extremely abundant, and because gifted guys from Fairchild figured out how to make it pure and how to produce semiconductors on industrial level. $\endgroup$ – andselisk Jul 10 '17 at 19:15
  • $\begingroup$ It's a little bit late where I currently am, I'll try my best to provide a more or less complete answer tomorrow, unless somebody else takes care of it. $\endgroup$ – andselisk Jul 10 '17 at 19:25