There is a lot of free space in air, not need of pushing anything away. Just to get into the picture, at normal conditions, molecules of air typically freely fly along a path several hundred times their size, until they hit other molecules.(higher tens of nm)
Imagine a large gym where is flying and bouncing 200 volleyballs, and you just add 6 basketballs. Do they need to push volleyballs away ? No, the do not.
What happens, when the molecule counts per volume increases, is the increase of pressure and the gas expansion. If you had 2 reservoirs of dry air, each 20 °C with the pressure 100 kPa, and if you let evaporate some water in one of them (while keeping the temperature), its pressure would increase. If they were interconnected, its content would partially expand to the other until pressures got equal.
Ať the given T and p, gas contains (idealized,with small error ) the same number of molecules, regardless of gas.(Avogadro law). If a molecule has a lower mass, the gas has lower density:
$$\rho=\frac{pM}{RT}$$
Air with vapor expands to have the same pressure as the surrounding air, so numbers decrease to value according to p and T.
Total number of molecules per unit volume is $\frac NV =\frac {N_\mathrm{A}.pV}{RT}\frac 1V=\frac {N_\mathrm{A}.p}{RT}$
Perhaps all started with misunderstood replacement. If you add 10 mmol of air molecules to 1 mol of air molecules do they replace them? The same for 10 mmol of water molecules.
This schematic illustration may help (not in the scale):
- The first line below illustrates 2 regions of dry air.
- The second line gets in the left extra water molecules, gaining higher density of molecules and therefore pressure.
- The third line is the result after higher presure of the left region pushing surrounding air away and expands until equalization of molecular densities and pressures.molecules, gaining higher density of molecules and therefore pressure.
At the same temperature and pressure, assuming ideal gas behavior, there is the same average space per a single molecule of a gas, regardless of the type of this molecule ($\ce{N2}$, $\ce{O2}$, $\ce{CO2}$, $\ce{H2O}$, $\ce{CH4}$,...)
..A.....A.....A.....A.. | ..A.....A.....A.....A..
..A..W..A.....A..W..A.. -->| ..A.....A.....A.....A..
..A.....W.....A.....A.....W.....A..| ..A.....A.....A.....A..
- A = nitrogen or oxygen molecules of air
- W = water molecules
- . = free space