[ Themodynamics Table of Contents ].

A P-T diagram is the most common way to show the phases of a substance. Figure 9 is the P-Tdiagram for pure water. A P-T diagram can be constructed for any pure substance. The line thatseparates the solid and vapor phases is called the sublimation line. The line that separates thesolid and liquid phases is called the fusion line. The line that separates the liquid and vaporphases is called the vaporization line. The point where the three lines meet is called the triplepoint. The triple point is the only point at which all three phases can exist in equilibrium. Thepoint where the vaporization line ends is called the critical point. At temperatures and pressuresgreater than those at the critical point, no substance can exist as a liquid no matter how great apressure is exerted upon it.