Physical Chemistry Chapter 7 The phase rule Phase diagram: shows the regions of pressure and temperature at which its various phases are thermodynamically stable Phase boundary: a boundary between regions, shows the values of p and t at which two phases coexist in equilibrium12 The Phase Rule Phase diagram: shows the regions of pressure and temperature at which its various phases are thermodynamically stable. Phase boundary: a boundary between regions, shows the values of P and T at which two phases coexist in equilibrium. Physical Chemistry Chapter 7