In crystalline solids, the constituent atoms sit close to each other in their equilibrium positions. The solid is not compressible because as the pressure is increased, the atomic orbitals tend to overlap and due to Pauli exclusion principle a large repulsive force comes into play.
Let's come to gases. Gases are easily compressible than liquids or solids. This is because the molecules of a gas are far apart and there is no significant overlap between the orbitals. It is also easier to compress it because there is a long-range Van der Waal attraction (the attractive part of the Lennard-Jones potential).
However, if we keep increasing the pressure on a gas, keeping the temperature fixed, there comes a time when a huge pressure needs to be applied to the gas to convert it to a liquid. Is it the same Pauli exclusion principle (PEP) that opposes compression during gas to liquid transition?
Do liquids resist compression due to the same reason i.e., Pauli exclusion principle (PEP) like solids?