# Does the vapor pressure of a substance depend on the presence of other gases?

The Clausius-Clapeyron equation (CC equation) can be used to find the (saturated) vapor pressure of a substance ie. the gas pressure at which the two phases (vapor + liquid or vapor + solid) reach equilibrium. However, what happens when the vapor is mixed together with other gases, such as water vapor mixed together with air? The reason I am asking this is because the derivation of the CC equation in my book relies on the assumption that the two phases are at equal temperature and pressure, whereas in the presence of other gases, the temperature of the vapor and liquid/solid will obviously be equal at equilibrium, but the pressure is not (since the other gases contribute with some pressure as well, and in general the partial pressure of the vapor will be smaller than the total pressure). This same problem arises if you consider the the equilibrium between, say, a solid and a liquid, so long as one of the phases is in a mix (like if the liquid is mixed with other liquids). If the CC equation changes as a result of this, then how does, say, the vapor pressure as a function of temperature change too?

• The net pressure changes, but the individual partial pressure of the vapour being looked at remains same, doesn't it?
– TRC
Jul 23 at 14:24
• @TRC Could you check out the comments I made under Chet Miller's answer? The issue is that in the derivation of the CC equation, you get a term involving the change in the pressure of the liquid (more specifically, a term like vdp, where v is the volume per particle of the liquid and dp is the change in pressure), which for most liquids around us should be zero (since the pressure of the liquid should remain at the ordinary atmospheric pressure). This would give a different result for the equation. Jul 23 at 14:53
• The vdp term for the liquid leads to the Poynting correction I referred to in my comment. For the vapor, even the pure vapor, the ideal gas law leading to the CC equation is not valid, and you need to use VdP for that too. Jul 23 at 19:18
• I'm not familiar with the derivation as of now - I will look it up and try to solve your question.
– TRC
Jul 24 at 3:26