Which of the following occur(s) when water vapor is added to a dry gas at a constant pressure?

1. The volume occupied by the gas mixture decreases.
2. The relative humidity of the mixture increases.
3. The partial pressure of the original gas is reduced.
a. 1 and 2 only
b. 1 and 3 only
c. 1, 2 and 3
d. 3 only

ANS: C
The dry volume of a gas at a constant pressure and temperature is always smaller than its satu-rated volume. The opposite is also true. Correcting from the dry state to the saturated state al-ways yields a larger gas volume. The pressure exerted by water vapor is independent of the other gases with which it mixes, depending only on the temperature and RH. Therefore, the addition of water vapor to a gas mixture always lowers the partial pressures of the other gases present.