# How does composition of air affect vapor saturation limit?

Suppose we hold temperature, pressure, and gas composition (say, a standard atmosphere) constant. Then we know that if we allow the water content of the air to vary it can range between 0 and some saturation limit S g/L.

How does that saturation limit vary with the composition of the air? For example, does it depend on the molecular weight and proportions of the other constituents of the air?

Assuming the water (or whatever substance is in question) does not chemically react with any other constituents at that temperature and pressure are there other constituent chemical relationships that can affect the saturation limit?

• To a first order approximation, no. But you seem to realize that if you start to consider the solubility of gas in the droplet of water at saturation, then there can be some effect on the vapor pressure. Try to define the question mathematically with a few equilibrium equations that include solubility constants of the gases in the mixture, and include a formula for the colligative effects, and you might get some interest in starting to develop an answer – repurposer Apr 28 '15 at 14:50
• @repurposer Are you saying there are solubility effects in gases? If so I'm not familiar with those. To clarify what I think you might be suggesting elsewhere in your comment: I'm interested in a "static" answer. I.e., I'm not interested in whether, at and beyond saturation, condensates can dissolve other gases and thereby reduce air pressure and revaporize or anything like that. Rather, the question is more generally something like, "Can air saturated with water vapor hold more or less alcohol vapor?" Or, "Can an atmosphere of nitrogen hold more or less water than one of helium?" – feetwet Apr 28 '15 at 15:46
• then, I think the answer is no, the partial pressures will be independent of one another and add to the total pressure by Dalton's Law. However, if you are looking at low temperatures or high pressures compared to atmosphere, or particularly polar, polarizable, or large molecules then dispersion forces will affect the overall equation of state. But I would not expect substantial differences to say that any gas would 'hold' much more of another gas, unless you are, say making detailed thermodynamic measurements, say of Virial coefficients for some mixture, which almost nobody does – repurposer Apr 28 '15 at 16:01
• @repurposer So in layman's terms, in a gas state (below any applicable chemical reaction temperatures) molecules don't care about the size or mass or composition of the other molecules they're bumping into. They only care about the temp ("avg speed") and pressure ("crowdedness"), and those two parameters determine each chemical's vapor saturation limit? But -- and this is worth an answer -- you suggest there is a point at which the chemical's size does come into play. And other chemical properties would change the answer -- polarization (not sure what that is outside of light)? Ionization? – feetwet Apr 28 '15 at 16:33
• I think I won't write a full answer right at this moment. Polar molecules have an electric dipole, polarizable molecules form instantaneous dipole (see dispersion). It's good that you bring up ions, which behave more like an electric monopole. I should back up a little and say, I think the layman's terms as you put it will get you in trouble, see this link, and you can see there really is a sizable vapor composition difference once you have condensed some liquid in a system – repurposer Apr 28 '15 at 16:50

$K_{p}=p_{H_2O}$, where $K_p$ is the equilibrium constant in pressure terms.
Consider this case similar to that of aqueous tension - it depends only on the temperature of the system, and not the other (non-reacting) components. Whenever we want to find the pressure of the dry gas, i.e. pressure of the mixture of gases $-$ the pressure due to the water vapour.