A couple of colleagues and I were talking about the nature of dewpoints, specifically the dewpoint of water in relation to the chemical composition of the atmosphere in a closed space.

If I remember my theory, then the chemical composition of the atmosphere should not have any impacts on the dewpoint of the water (assuming ideal gas law and that the atmosphere won't react with the water vapors)

My colleagues disagree with this, as they are of the opinion that different gasses can hold different concentrations of water vapor, e.g. $\ce{CO2}$ versus $\ce{N2}.$ As such, they argue that water would "crash out" of the gas phase at higher temperatures in atmospheres that could "hold less water".

Instead of wasting countless hours on research, I thought I might as well ask you brilliant people, if any of you could settle this dispute, or point me in a direction where an answer could be found.


1 Answer 1


Gases do not hold water vapor. Vapor is gas on its own. Neither oxygen nor nitrogen need other gases to hold them either. Gases are mutually toward each other just bystanders. Vapor can be present in vacuum alone.

The dewpoint is the temperature, at which would liquid water have the same saturated vapor pressure as the partial pressure of vapor present in the existing gaseous phase. If such vapor comes to contact with surface of temperature at or below the vapor dewpoint, part of vapor starts to condense, until the surface temperature is above the new dewpoint of vapor with lower partial pressure.

At special conditions near the liquid critical point, the dewpoint does depend on other gases, but in sense of total pressure. Or if there is strong interaction between gases. But neither of it is the case of water vapor in air near ambient conditions.


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