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Phase diagram contradiction: why does vapor exist?

  1. Oct 15, 2011 #1
    When you learn about phase diagrams of pure substances, you learn that the liquid and gas phases are in equilibrium only along the line separating the pure liquid and pure gas regions.

    But if you have a sample of liquid in a closed container with some empty space in it, that empty space eventually fills with vapor, which reaches equilibrium with the liquid phase. And this will happen at any p and T that are within the pure-liquid region of the phase diagram.

    So what is the correct way to interpret this observation using a phase diagram? They seem contradictory.
  2. jcsd
  3. Oct 15, 2011 #2


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    Where do you see contradiction?
    In equilibrium you have vapour and liquid, both at the same pressure and temperature - just on the diagram line.
  4. Oct 15, 2011 #3
    The contradiction is that the vapor+liquid coexist at p and T not on the equilibrium line.
  5. Oct 15, 2011 #4


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    Really? How did you get this???
    As they reach equilibrium they lay on equilibrium line.
  6. Oct 15, 2011 #5
    Fill a glass with water and cover it. The water starts to evaporate. It will evaporate until a characteristic "vapor pressure" of that compound is reached. For water this is about 24 mmHg at room temperature. There is now an equilibrium between liquid and vapor, at a pressure and temperature that are NOT on the phase diagram's coexistence line (i.e. the normal boiling point).
  7. Oct 15, 2011 #6


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    Partial pressure of water vapor is 24mm Hg. The rest of the pressure is caused by air.

    Phase diagram applies to single substance vapour/liquid equilibrium if they are observed in absence of other substances, and may be used as a good approximation (in most cases - it assumes no special forces between different gases) for partial pressure of the vapour and the liquid in presence of other substances.
  8. Oct 15, 2011 #7


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    Staff: Mentor

    The contradiction is that you ca't have "some empty space" at a chosen pressure. "Some empty space" can only mean a vacuum.
  9. Oct 15, 2011 #8
    Not so, Russ. At NTP the effective volume of the gaseous molecules is roughly 0.1% of the total volume. The rest of that volume is empty of all mass and is just as much a vacuum as you would find in outer space.
  10. Oct 15, 2011 #9


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