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Water (& Hydrogen Bond?)

  1. Dec 5, 2004 #1

    MR

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    I read that water expands when it freezes due to it's hydrogen bond. What on earth (or ocean, for that matter) does a hydrogen bond have to do with becoming less dense with freezing :confused: ?

    Please explain taking into consideration that I haven't taken chem. since 10th grade. Thanks!
     
  2. jcsd
  3. Dec 5, 2004 #2

    mrjeffy321

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    http://www.newton.dep.anl.gov/askasci/chem99/chem99245.htm

    Water expands when it freezes because of "hydrogen bonding."
    This means that the hydrogen on an H2O has a strong attraction
    for the "lone-pair", unbonded electrons on other nearby H2O
    molecules. In crystalline ice, each oxygen atom is surrounded
    by 4 hydrogen atoms (2 of its own and two from two other,
    neighboring water molecules in the crystalline lattice). This
    forms a "network structure" which is, incidentally, the same
    as diamond's (but with weaker bonds). The network structure has
    a lot of space between molecules. In fact, there is more
    space between molecules in this network structure than there
    is (on the average) in the liquid structure. Since there
    is more space between molecules in ice than in liquid water,
    ice is less dense.
     
  4. Dec 5, 2004 #3

    DaveC426913

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    Or, visually:

    Water: random orientation, molecules can get tightly-packed

    Code (Text):
    O-H  H  H  H
    |   /   |  |
    H  O  H-O  O-H
        \  
    H-O  H   H-O
      |    O   |
      H   / \  H
         H   H
    Ice: fixed orientation, molecules align, leaving large gaps
    Code (Text):
      O     O     O     O
     / \   / \   / \   / \
    H   H H   H H   H H   H
         O     O     O
        / \   / \   / \
       H   H H   H H   H

    Because of H2O's lopsided structure (an effect of the hydrogen bonds), its crystalline matrix is a very inefficent use of space. The looser-packed substance has a lower density, and floats.

    (BTW, this is just one of the many special properties of water that make it a staple substnane for the creation of life.)
     
  5. Dec 5, 2004 #4

    MR

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    Thanks!
    Is there anything else that gets less dense when it freezes?
     
  6. Dec 5, 2004 #5

    chem_tr

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    Gold Member

    According to Dave's explanation, anything that makes hydrogen bonding should show this "phenomena", for example ethyl alcohol may freeze like that, but since the freezing temperature is different (I remember that it should be around -80°C), and since one carbon is present rather than hydrogen, the orientation scheme may not be identical.
     
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