Water and pressure

  1. Hi!

    So I'm pretty confused about the effects that pressure has on water. If I increase pressure, I increase the boiling point. Likewise, if I increase pressure I increase temperature. How's that?

    That's the way air-conditioning works, by increasing pressure on one side and decreasing pressure on the other this way one side gets hotter and the other gets colder. But if by increasing pressure, temperature increases, shouldn't my deodorant bottle be hot?

    And if I have water and I apply pressure, shouldn't it turn to ice at some point? I'm so confused... Can anybody help me?
     
  2. jcsd
  3. SteamKing

    SteamKing 8,815
    Staff Emeritus
    Science Advisor
    Homework Helper

    You are confused about how air conditioning works. See this article (vapor compression cycle):

    http://en.wikipedia.org/wiki/Refrigeration#Vapor-compression_cycle

    A liquid boils when its vapor pressure equals or exceeds the ambient pressure acting on the free surface. Water boils at 100 C at sea level when the atmospheric pressure is 1 atmosphere. If you go to the top of a tall mountain, where the atmospheric pressure is less than 1 atmosphere, the boiling point of water is reduced below 100 C.
     
  4. russ_watters

    Staff: Mentor

    Welcome to PF!
    Anything that is hot can be made cold again: the effect isn't permanent. If it was, air conditioning wouldn't work. So with air conditioning, it goes like this:

    1. The gas is compressed and gets hot.
    2. The gas is run through a heat exchanger to cool it (and some liquefies).
    3. The gas is allowed to expand, cooling it.
    You'd expect that, yes, but ice has a crystal structure that makes it less dense than water. So it isn't that easy.
     
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