Dalton’s Law Applied to Air
We have seen that air is the mixture of various gases like nitrogen, oxygen, water vapor and other gases that don’t react chemically, but exists physically distinct, hence Dalton’s law of partial pressure is applicable to it. If we consider the air made up of dry air (without considering individual constituents of air) and water vapor, the total atmospheric pressure or the barometric pressure exerted by the air is sum of the partial pressure of the dry air and partial pressure of the water vapor.
Let us suppose that the total barometric pressure exerted by air is P, and the partial pressures exerted by the dry air and water vapor are Pd and Pv respectively, then:
P = Pd + Pv
Since the percentage of dry air is higher in this mixture, it is quite obvious that the partial pressure exerted by the dry air is much higher than that exerted by the water vapor. However, during the cooling or heating of the air, the dry air does not undergoes any change in phase, but the water vapor condenses on cooling and gets superheated on heating. The major changes that occur in the air during air conditioning process of heating or cooling the air occurs within the water vapor. Hence, though the quantity of water vapor in the air is very small (much lesser than 1%) compared to the others gases, it plays the major role in the psychrometric properties of the air. In fact most of the psychrometric studies are revolved around the water vapor present in air.