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I'm sure this is for most of you a basic question, but it really puzzles me:

How it is that, even though all materials expand as they get warmer, and contract (maybe these are not the correct terms) when get colder, water exapands when freezes.

Thanks a lot.

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    $\begingroup$ First, the thermal expansion coefficient for a given material is not a constant - it varies with temperature and can be both positive and negative depending on the temperature. Second, to add to the answer below, there are at least 4 elements that exhibit a solid phase at lower density than the liquid, namely silicon, germanium, antimony, and bismuth. Both silicon and germanium have the fairly open diamond lattice in the solid, yet are 8-12 fold coordinated metallic liquids. $\endgroup$
    – Jon Custer
    Commented May 13, 2015 at 13:24
  • $\begingroup$ Marked as duplicate but t just says there is a different lattice structure. No explanation as to why a different lattice structure forms with lowering of temperature. Is that just assumed knowledge, or something we just should accept ? $\endgroup$
    – phil soady
    Commented May 1, 2018 at 1:20

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The expansion upon freezing comes from the fact that water crystallizes into an open hexagonal form. This hexagonal lattice contains more space than the liquid state.

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  • $\begingroup$ And how do other materials freeze, or solidify? $\endgroup$
    – Masclins
    Commented May 13, 2015 at 11:55
  • $\begingroup$ how does that explain the change of lattice structure ? $\endgroup$
    – phil soady
    Commented May 1, 2018 at 1:11

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