How pressure affects partial pressures (and dewpoint)

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I have read numerous times that equilibrium vapor pressure (EVP) is a function ONLY of temperature. This at least partly makes sense to me (so I think) given energy of molecules and movement associated with such. But apparently this is not true for the partial pressures?

I once thought that even for partial pressures, it was only a function of temperature, but I am reading that system pressure has an effect.

See this source as one example: https://www.vaisala.com/sites/defau...d-air-Application-note-B210991EN-B-LOW-v1.pdf
"If this air is compressed and the total pressure is doubled to 2026.6 mbar, then according to Dalton’s law, the partial pressure of water vapor, e, is also doubled to the value of 5.6 mbar."

It is almost intuitive that pressurizing a system with increase the partial pressures, but on the other hand it feels strange to think that EVP would not be affected by pressure while partial pressure would, since EVP is just the 'maximum partial pressure for a given temperature.'
Furthermore, I thought that the ideal gas law in essence states that the pressure of one component is independent of the others. But clearly we are capable of compressing gases which compresses all partial pressures...

If I have this right, changing system pressure does not affect the maximum possible vapor pressure at that temperature, but it affects the actual pressure up to that maximum?
 

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  • #2
russ_watters
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It is almost intuitive that pressurizing a system with increase the partial pressures, but on the other hand it feels strange to think that EVP would not be affected by pressure while partial pressure would, since EVP is just the 'maximum partial pressure for a given temperature.'
You've discovered why air compressors need driers/drains. If the incoming air is close to saturated, pressurizing it will increase the vapor pressure above the EVP, and it will start to condense.
Furthermore, I thought that the ideal gas law in essence states that the pressure of one component is independent of the others.
That's Dalton's law.
But clearly we are capable of compressing gases which compresses all partial pressures...
That doesn't contradict Dalton's law, it follows Dalton's law.
If I have this right, changing system pressure does not affect the maximum possible vapor pressure at that temperature, but it affects the actual pressure up to that maximum?
Yes.
 
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  • #3
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Thank you Russ for that very clear and concise response. It made me realize I've framed my question incorrectly. Which perhaps still helps to answer my question.

Let me know if I go wrong:
Given a pan of water evaporating to it's surroundings (but defining some system boundaries), the partial pressure exists as a function of temperature ONLY (at least in theory if ideal) and not on the surrounding atmospheric pressure. Increasing total system pressure by increasing pressure of 'dry air' should not change the partial pressure of water vapor.

This is different than compressing ALL gases simultaneously, such as would happen in a compressor. In that case, the concentration of H2O molecules is being increased for a given space because the source air itself has H2O.

If I have this right so far, the next part is concerning dewpoint.
It is said dewpoint changes with pressure (I assume as a function of partial pressure changing).
For this to be true, it must be implied that the change in pressure brings with it an increase in H2O molecules, as if we are compressing the entire system with a pressurizing source that itself has H2O (which would make sense on earth).

Increasing pressure of the system by increasing some other non-water gas (such as 'dry air'), it would not itself change the partial pressure of water vapor. Is this right?
 
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"Changing the system pressure" is ambiguous. What you're describing in the example is increasing the pressure by compressing all of the gases, including the water vapor. So naturally the vapor pressure increases. But if you increased the system pressure by injecting dry air or some other gas the vapor pressure would be unaffected, at least in an ideal sense.
 
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  • #5
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injecting dry air or some other gas the vapor pressure would be unaffected, at least in an ideal sense.

JT you hit on the head my issue!
 
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Yeah, it's almost as if I anticipated your question.
 
  • #7
russ_watters
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Given a pan of water evaporating to it's surroundings (but defining some system boundaries), the partial pressure exists as a function of temperature ONLY (at least in theory if ideal) and not on the surrounding atmospheric pressure. Increasing total system pressure by increasing pressure of 'dry air' should not change the partial pressure of water vapor. [emphasis added]
No, the EVP (saturation pressure) is a function of temperature. You had the meaning of EVP correct previously.
It is said dewpoint changes with pressure (I assume as a function of partial pressure changing).
Dewpoint is a proxy for EVP/saturation pressure. It changes with temperature.
For this to be true, it must be implied that the change in pressure brings with it an increase in H2O molecules, as if we are compressing the entire system with a pressurizing source that itself has H2O (which would make sense on earth).
Increasing temperature for a mixture in equilibrium causes more water to evaporate, to keep it in equilibrium. This will then change the ratio of the gases in the mixture. It is very much not like compressing the entire system.
Increasing pressure of the system by increasing some other non-water gas (such as 'dry air'), it would not itself change the partial pressure of water vapor. Is this right?
Correct! Phew! Overall you had more right in your first post! You basically inverted the terminology.
 
  • #8
russ_watters
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Dewpoint is a proxy for EVP/saturation pressure. It changes with temperature.
Wait, sorry I had the tail wagging the dog there. Dewpoint changes with the amount of water in the air, so yes, it also changes with vapor pressure.
 
  • #9
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Russ, I was writing a long response tearing you on that dewpoint :-p, glad I saw your update.

I do feel like some lines are getting crossed on this one. It is surely my fault as I do not speak in proper and precise scientific terms.

No, the EVP (saturation pressure) is a function of temperature. You had the meaning of EVP correct previously.
Yes, I still understand this much is true for EVP. I'm not discussing that though. Partial pressure is a state of the system. In flux. I understand that, and so my phase was wildly inaccurate in that sense.
What I am saying is the variables that can alter the rate of change is temperature (and other things like the physical distribution of the H2O molecules) but not the other partial pressures of the system (i.e. atmospheric pressure). This changes insofar as reducing atmospheric pressure can induce boiling which certainly increases the rate of evaporation.
Maybe all I'm getting at here is a basic acknowledgment of Dalton's law? Or maybe I'm just butchering the approach.

Increasing temperature for a mixture in equilibrium causes more water to evaporate, to keep it in equilibrium. This will then change the ratio of the gases in the mixture. It is very much not like compressing the entire system.
Huh? My statement was not about increasing temperature, it was about increasing pressure. As in, take atmospheric air and compress it. I'm missing some connection here maybe.
 
  • #10
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For context, the original notion I was trying to 'confirm' or 'debunk' here is that "dewpoint depends on atmospheric pressure."

My thinking is that it does, but only insofar as more H2O is added to the system. If one increased only the dry component of atmospheric pressure, it would not change (is my current thinking).
 
  • #11
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Yes. Or if you removed dry air. You could remove all of it and it wouldn't change the dew point.
 

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