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Molecular phase shift in a non-enclosed area – how ?

  1. Oct 4, 2011 #1
    I have read this and to put it mildly I am confused, so maybe someone can clear this up.
    The dew point is defined as the point/temperature at which the Partial Pressure of the water vapour within the air packet equals the Saturated Vapour Pressure.

    I understand that the dew point is a temperature at which H2O will phase shift from gas to liquid and this will alter with the combined pressure (eg, air). I understansd the Partial Pressure is (in this case) the pressure of the H20 with in the air and is not connected to the other partial pressures of other gasses.

    But. I thought (maybe wrong) the Saturated Vapour Pressure was the escaping molecular pressure against the inside of a closed container.

    If this is true then how the Dew Point exist in a non-enclosed container, ie. The atmosphere.
    or - in other words
    How can the partial pressure (the outwards force of a packet of gas) be equal to the Saturated Vapour Pressure (which can't exisit in the atomosphere)

    Thank you
    Last edited: Oct 4, 2011
  2. jcsd
  3. Oct 4, 2011 #2
  4. Oct 4, 2011 #3
    Thanks for the response.

    I do understand the various phases of water and partial pressures

    But (there is always a but) the thing that is confusing me is how can a non-enclosed space have a saturated vapour pressure (which, by deffinition MUST be enclosed). Should the atmosphere not by definition have an unlimited pressure possible ? - Note I am using the word "unlimited" as a generalisation

    As an example, in a sealed container the SVP would be achieved when the molecular escape pressure = the maximum abortion possible by the liquid surface. So the vapour pressure “from” the surface = the pressure “into” the surface.

    If this is true (?) then a dew point should not be possible in the (non-enclosed) atmosphere. But of course clouds do form, so I am misunderstanding something really basic here.

    If (for example) the humidity reaches 100% at the temperature required to phase shift (gas to liquid) – of course assuming the presence of nucleation substances – droplets will form. But as it’s a non-enclosed area the humidity should simply not reach 100% as the SVP can not be attained - but of course it does ;-)

    I have been studying this for days and am still no further along with understanding it
    Last edited: Oct 4, 2011
  5. Oct 4, 2011 #4
    Hi again Mozil

    It’s just come to me.

    I am thinking too big (macro and not micro)

    In the atmosphere SVP describes the partial pressure over a flat surface and not an enclosed area. Of course the surface of a droplet is flat (apart for the electron lone pairs) on a molecular scale.

    Thanks for your input – it focused my brain in the right direction.
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