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

[karen_lorr]

Hi
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

 

[Mozil]

Nice question IMHO... Frankly, it is not so simple to find out it. But could you have a look at those pages for the beginning:
http://en.wikipedia.org/wiki/Water_vapor
http://en.wikipedia.org/wiki/Saturated_vapour_pressure
You welcome if any further...

 

[karen_lorr]

Hi
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

 

[karen_lorr]

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.

 

[pmacias]

for your question. I can understand your confusion about the concept of dew point and its relationship to partial pressure and saturated vapor pressure. Let me try to explain it in a simple way.

First, let's define some terms. Dew point is the temperature at which water vapor in the air will condense into liquid water. Partial pressure is the pressure exerted by a specific gas within a mixture of gases. And saturated vapor pressure is the pressure exerted by a vapor when it is in equilibrium with its liquid form at a certain temperature.

Now, to answer your question about how dew point can exist in a non-enclosed area, we need to understand that the atmosphere is not a completely open space. It is a mixture of different gases, including water vapor. The water vapor molecules are constantly moving and colliding with each other and with other gases in the atmosphere. This creates a certain pressure, which is the partial pressure of water vapor.

As the temperature of the air decreases, the water vapor molecules will start to move slower and have less energy. This will reduce the pressure exerted by the water vapor, but it will also decrease the saturated vapor pressure. Eventually, at a certain temperature, the partial pressure of water vapor will equal the saturated vapor pressure, and the water vapor will start to condense into liquid water. This is the dew point temperature.

So, to summarize, the dew point can exist in a non-enclosed area because the atmosphere is not an open space and the water vapor molecules in it can still exert pressure. The concept of partial pressure and saturated vapor pressure helps us understand the conditions at which water vapor will condense into liquid water. I hope this explanation helps to clear up your confusion.