Phases
Depending on the conditions, e.g., the values of pressure and temperature, a substance assumes different phases—solid, liquid, vapor—which can also coexist. We shall need property relations for all individual phases as well as for the coexisting states.
Atoms and molecules interact through interatomic potentials φ (r) of the form depicted in Fig. 6.1. For intermediate particle distances around d, the particles attract each other, while they repel each other when they are pushed very close together (r < d). For large distances (r » d), the particles do not notice each others presence (φ (r) → 0 for r → ∞).
In a solid, the particles sit at fixed locations in the atomic compound, e.g., a crystal lattice, and oscillate around the minimum of the potential. The interatomic forces are strong, and keep the solid together.
When the temperature is increased, the oscillations become stronger, and the particles have enough energy to split the molecular bonds with their neighbors, while the attractive forces are still significant. The particles can move freely, but are densely packed with distances close to d. This is the liquid state.
At even higher temperatures the particle energies exceed the attractive potentials which cannot hold the particles together anymore. The particles move fast at greater average distances. This is the gaseous, or vapor, state.
In solid and liquid states, the particles are in permanent contact and interaction. While gas particles have a large average distance, they nevertheless interact through frequent collisions. The interaction between particles leads to microscopic exchange of energy and momentum which facilitates the macroscopic transfer of energy and momentum. The constant redistribution of momentum and energy between particles drives the system towards the equilibrium state.