Phases and Equilibrium

The phases of matter represent classes of the type of molecular motion found at different temperatures.  The 'motion' of the maolecules is described by the position and momenta of the atomic nuclei. The position of the molecule as a whole is derived from the center of mass of the atoms that compose it  The molecules undergo classical (not quantum mechanical) motion, either harmonically bound (solid), chaotically quasi-bound (liquid) or unbound (gas). More here.

Courtesy: PJ Brucat, University of Florida