The three phase equilibrium curves meet at the triple point. At the triple point, all three phases (solid, liquid, and gas) are in equilibrium. Since the triple point is a point, there is only one temperature and one pressure where the three phases will exist. This fact often helps in identifying compounds or in problem solving.

The critical point is the highest temperature and pressure at which a pure material can exist in vapor/liquid equilibrium. At temperatures higher than the critical temperature, the substance can not exist as a liquid, no matter what the pressure.

At temperatures and pressures higher than the critical point, the substance is considered a fluid--something neither gas or liquid. At pressures lower than the critical pressure (but at higher temps), the substance is considered a gas.

Calculations in 10.213 do not usually involve temperatures past the critical temperature.