A theoretical temperature-specific volume-molar fraction (T-v-x) phase diagram of the binary systemA-H2O presenting a hydrateH with congruent melting is studied. The phase diagram is divided into two subsystems by the one-component phase diagram of
the hydrateH. The eutectic invariant planes which occur in both subsystems are described. The relationships between the invariants are
Isochoric and isoplethal theoretical sections are given. The one-component phase diagram of the hydrateH is described. It is pointed out that the triple line solid-liquid-vapour which relates the congruent melting is not necessarily
perpendicular to the molar fraction axis. The means that the three phases, solid, liquid and vapour, have not necessarily
the same composition.
TheT-v-x representation gives an explanation to the deviation between the maximum of the melting curve and the stoichiometric composition
of the hydrate which is often observed in theT-x binary diagramA-H2O.