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Vapour Pressure and External Pressure

  1. Jun 25, 2006 #1
    When the vapour pressure of a liquid equals to the external pressure, the liquid will boil.
    So, what if a liquid is placed in a vacum? Will the liquid boil instantly when heated?
  2. jcsd
  3. Jun 25, 2006 #2


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    Staff: Mentor

    It depends on the liquid and the temperature, but many will boil at room temperature in a vacuum.
  4. Jun 25, 2006 #3
    Temperature is a measure of the average kinetic energy of molecules.

    A Vacuum is devoid of all matter by definition and therefore has no temperature.

    If your substance exists as a liquid state in a vacuum, then it will not boil; however, if it exists as a gas in vaccum, then it will boil. (Actually, boiling occurs because of microscopic bumps on the pot that agitate the liquid. It would probably turn to vapor directly without boiling in outer-space)

    You won't have to add heat to achive this, it will occur spontaneously.
    Last edited: Jun 25, 2006
  5. Jun 26, 2006 #4
    How do we know whether a substance will exist in liquid or gas form in vaccum?
  6. Jun 26, 2006 #5
    Vapor pressure of a liquid (or even a solid) is a function of temperature.
    For sure, if it is positive the substance will evaporate when placed in vacuum.
    This doesn't mean that this will happen like in a boiling pan.
    The details will play a role: is it in microgravity of not, is the temperature uniform or is there a temperature gradient, is the substance in contact with some other material, ...
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