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Boiling point

  1. Jan 12, 2008 #1
    [SOLVED] boiling point

    1. The problem statement, all variables and given/known data
    All of my sources say that a liquid boils when its saturated vapor pressure reaches the external pressure on the liquid. None of my sources actually explain why that makes sense.
    When they say "external pressure," does that include the vapor pressure of the liquid on the liquid itself?
    I just don't understand why the external pressure on the liquid from the gas above it cannot be greater than the vapor pressure of the liquid.

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 13, 2008 #2
    boiling means that the example is going from liquid phase to gas phase in the whole sample, (sorry for my English, hope you´ll understand). When they say external pressure then they mean, for example, atmospheric pressure which is 1atm at seal level. When the vapor pressure reaches to external pressure then the liquid becomes sufficient to overcome atmospheric pressure and lift the liquid (bubbles form inside the example). If the external pressure is higher than vapor pressure then those bubbles will not form. The example might vaporize very rapidly, but it won´t boil. Hope this will help you a little.
     
    Last edited: Jan 13, 2008
  4. Jan 13, 2008 #3
    I think you might want to look up the word "example". I think the word you want is "sample". The word "example" was used correctly only in the second sentence.

    Anyway, can you explain what it means that "the liquid becomes sufficient to overcome atmospheric pressure and lift the liquid"?
     
  5. Jan 13, 2008 #4
    When the external pressure is greater than vapor pressure then the bubble formed in the sample(Thanks for mentioning my mistake before) will collapse. But if the external pressure is equal or lower than the vapor pressure, the bubbles will remain(and expand) because the pressure inside the bubble is same as vapor pressure and grater or equal to the external pressure.
     
    Last edited: Jan 13, 2008
  6. Jan 13, 2008 #5
    That makes sense. Thanks.
     
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