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Boiling point Atmospheric/hydrostatic pressure

  1. Sep 26, 2014 #1
    So I understand that you get boiling when the vapor pressure equals atmospheric pressure so that the vapor within the medium can overcome the atmospheric pressure pressing on the surface of the liquid, but how do the bubbles overcome the hydrostatic pressure from the water above the bubble. If the vapor in the bubble is only at atmospheric pressure, wouldn't it collapse under the weight of the water above it? Is it because, if being boiled over a stove-top, the temperature source is heating the vapor in the bubbles to a pressure above atmospheric? If so, how come bubbles still form under the surface when you are boiling under a vacuum at room temperature, where the liquid is actually getting colder?

    Thanks
     
  2. jcsd
  3. Sep 26, 2014 #2
    If the depth of the water in the pot is 8 cm, how much hydrostatic pressure would that correspond to? If we add that hydrostatic pressure to atmospheric pressure at the surface, what would the boiling point be?

    Chet
     
  4. Sep 26, 2014 #3
    Thanks for the reply.
    roughly 800 pascals right? Which is tiny, but is that being compensated for by an increase in pressure within the bubble or is the bubble allowed to compress while it floats up, though at a lower magnitude the higher the bubble goes.
     
  5. Sep 26, 2014 #4
    Now, let's round that off to 1000 Pa. So, if the equilibrium vapor pressure of water is 101kPa at 100C, what temperature is required for the equilibrium vapor pressure to be 102 kPa? If the bubble forms at the bottom of the pot, the pressure inside the bubble will be102kPa, and it will form at the slightly higher temperature. It won't compress as it rises up, but it might expand a little as the pressure drops.

    Chet
     
  6. Sep 27, 2014 #5
    Hmm I see, however, how do you get that extra pressure in the bubble when there exists no external heat source when boiling in a vacuum?

    Thanks
     
  7. Sep 27, 2014 #6
    When boiling in a partial vacuum, you have reduced the pressure in the chamber to a value equal to the equilibrium vapor pressure of the liquid at its temperature in the vessel. Suppose that the temperature of the liquid water in the vessel is 25C. What is the equilibrium vapor pressure of water at 25C? This is the vacuum pressure you need to supply to get the water to boil. The heat to form the bubbles is supplied by the surrounding liquid, and as boiling continues, the liquid cools. This means that you need to keep reducing the vacuum pressure even further to allow boiling to continue.

    Chet
     
  8. Sep 27, 2014 #7
    Ah I see, I fully understand now. Thank You for continuing to follow up with my question.
     
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