  The main thermodynamical properties of the first order phase transition of
the relativistic mean-field (RMF) hadronic model were explored in the isobaric,
the canonical and the grand canonical ensembles on the basis of the method of
the thermodynamical potentials and their first derivatives. It was proved that
the first order phase transition of the RMF model is the liquid-gas type one
associated with the Gibbs free energy $G$. The thermodynamical potential $G$ is
the piecewise smooth function and its first order partial derivatives with
respect to variables of state are the piecewise continuous functions. We have
found that the energy in the caloric curve is discontinuous in the isobaric and
the grand canonical ensembles at fixed values of the pressure and the chemical
potential, respectively, and it is continuous, i.e. it has no plateau, in the
canonical and microcanonical ensembles at fixed values of baryon density, while
the baryon density in the isotherms is discontinuous in the isobaric and the
canonical ensembles at fixed values of the temperature. The general criterion
for the nuclear liquid-gas phase transition in the canonical ensemble was
identified.
