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Vapor Pressure

  1. Dec 4, 2005 #1
    Can anyone help to explain what vapor pressure does...i know what it is...but how can it be used

    if you put a cube of ice at 0 degrees C, in a vacumed container (amospheric pressue equals zero) wont the ice all go to gas, since its vapor pressure is greater than the atmospheric pressure? The way i understand it now, and dont know if this is the correct way of thinking of it is that vapor pressure is like how hard the compound is trying to turn to a gas and balance out the pressure, on the surface.

    any help appreciated

  2. jcsd
  3. Dec 4, 2005 #2


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    Gold Member

    The vapor pressure is the pressure of the vapor that exists in equillibrium with another phase at some given temperature. If the external pressure falls below the vapor pressure, then the other phase will go completely to vapor. To answer your first question: yes, you can boil ice at 0°C if you lower the pressure enough. To answer your second question: that's a somewhat correct way of thinking about it in the sense that substances with higher vapor pressures will tend to go to gas more easily and give off vapors at a faster rate. And the giving off of vapors will tend to increase the pressure and for a system at constant volume. But it is not really 'trying to balance' the pressure since there is nothing to balance it against. It will tend to bring the pressure of a system at constant volume closer to the vapor pressure, bu that isn't exactly balancing.
  4. Dec 4, 2005 #3


    that makes sense
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