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Liquid in a Vacuum

  1. Apr 4, 2010 #1
    Are there any substances that are liquid in a vacuum? It seems mercury should be liquid.

    If so, what forces keep it from solidifying, yet not disipating into a gas?
     
  2. jcsd
  3. Apr 4, 2010 #2

    Mapes

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    The equilibrium state in a vacuum is gaseous; no substance is immune. So it's just a matter of time and rates. Water evaporates before mercury, which evaporates before titanium, for example. (Assuming a large vacuum and small amounts so that vapor pressure and gravity are negligible.)
     
  4. Apr 4, 2010 #3
    I would have thought that if a substance went into a vacuum then by definition it would no longer be a vacuum.
     
  5. Apr 4, 2010 #4
    For solid to gas, whats the time frame we're looking at? Evaporating satellites?
     
  6. Apr 4, 2010 #5

    Mapes

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    For ice just under 0C in my freezer, a time frame noticeable by humans.
    For metal satellites, not a time frame noticeable by humans. :smile:

    I saw a beautiful chart of vapor pressure vs. temperature for different materials in a microfabrication class; I'll see if I can find the reference, or it may be findable online with some searching. Generally, it showed the trends you'd expect: vapor pressure dependent on temperature (since atomic detachment from a surface is a thermally activated process), lower vapor pressure for high-density, high-melting-temperature materials.
     
  7. Apr 5, 2010 #6
    I was actually wondering about the bulk of the fluid. What keeps it stuck together, yet not stuck together so tightly that it's a solid?
     
  8. Apr 5, 2010 #7
    This is due to evaporation where some of the molecules gain enough energy to overcome the intermolecular forces, break through the surface and escape.ln a closed container this will result in a saturated vapour which reaches an equilibrium(rate of evaporation being equal to rate of condensation).The saturated vapour pressure increases with temperature.For mercury the intermolecular forces are fairly "large" this being one reason why mercury is suitable for barometers.
     
  9. Apr 5, 2010 #8

    Mapes

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    Liquids are a trade-off between enthalpy and entropy. There's a tendency for the atoms (or molecules) to bond, but they're not very strongly bonded, so they rearrange frequently. If they were more strongly bonded, they would hardly rearrange at all; they would form a solid. If they were less strongly bonded, they would rarely even be attached; they would form a gas.
     
  10. Apr 6, 2010 #9
    OK. If I would have thought about it in the beginning, correctly, I would have gotten it. All liquids have a surface tension, i.e. volumentric tension (a force per unit area) that will hold them together without being under pressure.

    It seem, then, that so called surface tension is a direct result of any occasional bonding that occurs between molecules. Does that sound right?

    ---Come to think if it, I completely forgot about heat kinetic energy. Over a particular temperature would a glob of liquid distrupt, all at once?
     
    Last edited: Apr 6, 2010
  11. Apr 6, 2010 #10

    Mapes

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    Pretty much. Surface tension is just an energy penalty for creating additional surface area. If the atoms or molecules tend to bond to form condensed matter, then their minimum-energy configuration is one that minimizes surface area (here I'm ignoring subtleties like anisotropy).

    For the temperature at which a liquid is disrupted: are you talking about the boiling temperature?
     
  12. Apr 8, 2010 #11
    I suppose so. I hadn't really thought about it. This seems to be the temperature at which the average kinetic energy exceeds the average negative bonding energy... I'm sure you can state it better.

    Btw, you are aware that surface tension is something of a misnomer. The entire blob of liquid pulls together. The most noticeable effect is how the surface is becomes shaped due to the imbalance of forces at the boundary--thus 'surface tension'.
     
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