At low pressures, when the sample occupies a large volume, the molecules are so far
apart for most of the time that the intermolecular forces play no significant role, and
the gas behaves virtually perfectly. At moderate pressures, when the average separation
of the molecules is only a few molecular diameters, the attractive forces dominate
the repulsive forces. In this case, the gas can be expected to be more compressible than
a perfect gas because the forces help to draw the molecules together. At high pressures,
when the average separation of the molecules is small, the repulsive forces dominate
and the gas can be expected to be less compressible because now the forces help to
drive the molecules apart.