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Thread: Why does water expand when it freezes?

  1. #1
    Administrator CatInASuit's avatar
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    Default Why does water expand when it freezes?

    So, the usual science message is that as things got hotter, they expand and as they get colder, they contract.

    So why, if I freeze water, does it expand?

    Which part of science explains why water is different, or do all liquids act like that and I just haven't noticed?
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  2. #2
    my god, he's full of stars... OneCentStamp's avatar
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    Water expands when it freezes for the same reason a house made of Tinkertoys takes up more room than the same amount of Tinkertoys just lying unassembled in the box; water molecules formed into orderly crystals take up more room than the same molecules passing (more or less) freely by each other.
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    Stegodon kk fusion's avatar
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    Water molecules are dipoles, where the electric charge is concentrated on the oxygen end of the molecule. In liquid water, molecules form hydrogen bonds, which are weak bonds of the hydrogen ends with the free electrons of another molecule.

    Now if water is cooled down, the hydrogen bonds are frozen into place, pushing the molecules farther apart and reducing the density of the water. Because of the small size of water molecules, these effects override those of Brownian motion, so water decreases in density when cooled below a certain temperature (around 4 deg C).

    According to wiki there are other elements and compounds that show such anomalies of density, but water seems to be the only substance where these effects occur under natural conditions.

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    Content Generator AllWalker's avatar
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    Each water molecule is V-shaped, where two hydrogen atoms are attached to a central oxygen atom, hence H20. The way water is, the charge on each hydrogen atom is opposite to that on the oxygen, and so the oxygen on one water molecule might be attracted to a hydrogen on another - this is called an intermolecular bond.

    The hotter a molecule is, the more it vibrates. So has a liquid, the water molecules are vibrating hard enough that no lasting intermolecular bonds may form. A bunch of V-shaped objects vibrating can pack quite closely to each other. But as the liquid cools enough to solidify, the vibrations weaken enough that the intermolecular bonds start to influence the molecules more. Instead of packing in tight, the Vs start locking into a shape where the bottom "point" of a V is sitting close to one of the upper "end" points.

    Water does this because it is tiny, yet has incredibly powerful hydrogen-oxygen bonding. Most molecules that are water's size are gas well below room temperature, but this attraction keeps water a liquid.
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  5. #5
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    Just think, if ice was denser than water, the frozen ice in a pond would sink to the bottom and kill all the fishies because the fish would be trapped between the ice and mud on the bottom of the pond.

    As it is, my ice cube floats in my bourbon and that's nice. It makes a nice little tinkle when I shake the glass.
    Last edited by Duck Quack's Echo; 28 Dec 2009 at 07:47 PM.

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