  We derive a class of equations of state for a multi-phase thermodynamic
system associated with a finite set of order parameters that satisfy an
integrable system of hydrodynamic type. As particular examples, we discuss
one-phase systems such as the van der Waals gas and the effective molecular
field model. The case of N-phase systems is also discussed in detail in
connection with entropies depending on the order parameter according to
Tsallis' composition rule.
