I'm confused about which phases are considered compressible and which are considered non-compressible. In classical thermodynamics, gases and liquids are considered compressible. This is what the state principle (2 independent properties define all other properties) applies to gases and liquids. Am I correct in assuming solids are incompressible and thus the state principle doesn't apply to solids? Because we never study solids in my classical thermodynamics class. In fluid mechanics, we often treat liquid as incompressible. For this reason, we usually assume the density of the fluid doesn't change as the pressure changes. Can someone clear this up for me. Thanks!