View Single Post
Old 09-30-2019, 12:12 AM
JWT Kottekoe is online now
Join Date: Apr 2003
Location: California
Posts: 1,013
Originally Posted by am77494 View Post
That is also an incorrect understanding. Equilibrium happens with respect to fugacity not partial pressures. It just happens that the partial pressure of water vapor is approximately the same as its fugacity at low pressure.
At the low partial pressures relevant to this discussion, the vapor pressure is almost precisely equal to the fugacity. That is why it is an excellent approximation to consider water to be an ideal gas, and certainly for purposes of explaining it to your grandmother, the ideal gas approximation should capture the essential behavior. It is very misleading to attempt to explain it by invoking the volume of the gas molecules, when the essential physics is much simpler and has nothing to due with the properties of air.