 Accordingly, there will be three fusion or freezing point curves AC, BE and CDE for the three solid phases.In the phase diagram, we can notice that the top point D of the phase diagram is the congruent melting point of the compound AB because the solid and liquid phases now have the same composition.Evidently, at this temperature, the two-component system has become a one-component system because both solid and liquid phases contains only the compound AB.