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Why does water expand when cooled?

And are there any other substances which do so?

Update:

Gah I knew I made a mistake. I meant frozen =p

3 Answers

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  • 10 years ago
    Favorite Answer

    Water contracts when cooled, but it expands when frozen, because the molecules re-arrange themselves into a crystalline structure. I think other substances do this too, but I'm just guessing, I'm not sure.

  • 10 years ago

    Ice I (stable form at °C and 1 atm) has a diamond type lattice (C atoms form an infinite lattice where each C atom is tetrahedrally bound to four others) . In ice the C-C bonds are replaced by O-H...O or O...H-O bonds where the H...O represents a H bond. It is a rather open structure and ice has a density of 0.93 g cm^-3. What happens on melting is still a matter for discussion, but it is thought that about a quarter to a third of the H bonds break and the structure partially collapses (I view as like a building scaffold partly coming apart). (H2O)n ( n= 2, 3, 4, 6 etc) molecules form which occupy less volume than ice and consequently the density increases to 1.00 at ~4°C.

    This phenomenon where the liquid is more dense than the solid at mp is rare, but is observed in Si, Ge, Ga and Bi. (Google to confirm.) (Interestingly, Si and Ge have diamond lattices.)

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