  It was shown that in the canonical ensemble the simple exactly soluble
statistical model of nuclei decay into nucleons, which is a limiting case of
the statistical multifragmentation model, predicts the nuclear first order
phase transition associated with the Helmholtz free energy different from the
first order phase transition of the liquid-gas type associated with the Gibbs
free energy. The main thermodynamic properties of this phase transition were
explored on the basis of the method of the thermodynamic potential and its
first and second derivatives in the thermodynamic limit. It was established
that the thermodynamic potential $F$ is a piecewise smooth function and its
first order partial derivatives with respect to variables of state are
piecewise continuous functions. At the points of phase transition, the energy
in the caloric curve is discontinuous at the constant temperature and fixed
values of the specific volume, while the pressure and the chemical potential in
the equations of state are discontinuous at the constant specific volume and
fixed values of the temperature.
