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Vapor pressure of boiling water

  1. Jun 28, 2013 #1
    Why does the answer say that vapor pressure only depends on temperature and intermolecular forces substance experiences? I thought external pressure did affect vapor pressure because when you heat a pot of boiling water at higher elevation the boiling point decreases, and vapor pressure increases, due to less atmospheric (external pressure). What am I missing?

    Question
    Inside a half-filled water balloon at 25°C and sea level, the vapor pressure of water is 24 mm Hg. What will the vapor pressure of water in the balloon be if a diver takes it to a depth where temperature is 25°C and pressure is 2 atm?

    B.

    24 mm Hg


    Explanation:
    B. The vapor pressure of a substance depends only on the temperature and the intermolecular forces that substance experiences. In particular, it does not depend on external pressure. Therefore, the vapor pressure of water will not change and 24 mm Hg is the correct answer.
     
    Last edited: Jun 28, 2013
  2. jcsd
  3. Jun 28, 2013 #2

    DrDu

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    Science Advisor

    You are mixing something up here. The boiling point decreases, that´s true. At the boiling point the vapour pressure is equal to the external pressure. At higher elevation, the external pressure is lower and hence boiling occurs at a lower pressure. This is not due to the boiling pressure changing with external pressure.

    However you are right in that the vapour pressure also changes with in principle external pressure. This effect is quite small and you need high pressures to observe it.
     
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