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Amorphous ice is less dense than crystalline, why?

  1. Oct 9, 2005 #1

    Mk

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    http://www.lsbu.ac.uk/water/amorph.html puts the densities of various forms of amorphous ice at densities that are very low. g/cm-3 in fact. Is this a typographical error? If not why is it so dense? Crystalline ice's densities are attributed to all the empty space in-between crystals, amorphous ices have much less empty space.
     
  2. jcsd
  3. Oct 9, 2005 #2
    And this is based on what?
     
  4. Oct 9, 2005 #3
    Hi Mk,

    Imagine you have to fill a given box with small rigid cubes. Firstly you trow (fast and randomly) the cubes in the box and secondly you arrange them one by one.
     
    Last edited: Oct 9, 2005
  5. Oct 9, 2005 #4

    Mk

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    Which one is the amorphous and crystalline? I'm assuming the first is amorphous, that's why its dess dense?
     
  6. Oct 9, 2005 #5

    Gokul43201

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    According to the article, there's LDA and HDA. Which are you asking about ?
     
  7. Oct 10, 2005 #6

    PerennialII

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    So amorphous is a bit more dense than crystalline, if remember its density about right ... the differences are of the order of some tens or percents depending on what forms of ice in question .... doesn't appear a typo to me.
     
  8. Oct 10, 2005 #7
    I would have thought that amorphous (having no crystalline structure) has a larger opportunity for trapping air, whereas crystalline being geometric contains a fixed amount.

    ie, the crystalline ice traps air between the intermolecular (H) bonds, and as such has a fixed density, while amorphous ice has a random element (in molecular alignment) leaving extraneous spaces between said molecules.

    The analogy above is intuitive - being a random pile (amorphous) allows more gaps so more air and thus less density, compared to a stack with no gaps (crystal)...
     
  9. Oct 10, 2005 #8

    Gokul43201

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    But then, you can also compress amorphous ice and find stable configurations that lie on the other side of an energy barrier, enabling a higher final density.
     
  10. Oct 11, 2005 #9
    True, but that doesnt exclude the possibility of amorphous ice having a lower density up until it is compressed. :D

    How much can crystalline ice be compressed before it loses its structure?
     
  11. Oct 11, 2005 #10

    russ_watters

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    This issue is best dealt with from the standpoint of engineering or chemistry. The issue is one of crystal structure and what, exactly, different structures look like. HERE are the basic crystal structures. Unfortunately, I couldn't find a link in 10 seconds where they showed a picture with the packing factor listed next to it, so HERE is another site showing packing factors of various crystals. It is important to note that the packing factor is the same for all materials. It is the geometric relationship between the number of particles and the volume.

    Amorphous is tougher than crystalline because the density can vary, but as a general rule, ordered packing is more efficient than disordered, thus crystalline is always (afaik) more efficiently packed than amorphous. The reason has been said above: in amorphous, you get semi-random arrangements that often cause large voids.
     
    Last edited: Oct 11, 2005
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