A phase diagram for a typical pure substance in the pressure (p) temperature (T) plane is sketched below.
Figure 2.2: Pressure-temperature phase diagram of water (not to scale).
There are three equilibrium phases, the solid (s), liquid (l), and vapor (v) (or gas), with phase boundaries separating the phases. Crossing the phase boundaries results in a phase transition from one phase to the other. The liquid-vapor phase boundary (often called the vaporization curve), terminates in a critical point at sufficiently high temperatures and pressures. At temperatures above the critical temperature or pressures above the critical pressure there is no distinction between the vapor and liquid phases, and we have simply a single fluid phase. Indeed, at a microscopic level there is little difference between a liquid and a vapor; there are stronger interactions between the particles in the liquid, but there are no fundamental differences in symmetry between the two phases. Our description of fluid mechanics will apply equally well to liquids and vapors (gases).