  We study the effect of periodic boundary conditions on chiral symmetry
breaking and its restoration in Quantum Chromodynamics. As an effective model
of the effective potential for the quark condensate, we use the quark-meson
model, while the theory is quantized in a cubic box of size $L$. After
specifying a renormalization prescription for the vacuum quark loop, we study
the condensate at finite temperature, $T$, and quark chemical potential, $\mu$.
We find that lowering $L$ leads to a catalysis of chiral symmetry breaking. The
excitation of the zero mode leads to a jump in the condensate at low
temperature and high density, that we suggest to interpret as a gas-liquid
phase transition that takes place between the chiral symmetry broken phase
(hadron gas) and chiral symmetry restored phase (quark matter). We characterize
this intermediate phase in terms of the increase of the baryon density, and of
the correlation length of the fluctuations of the order parameter: for small
enough $L$ the correlation domains occupy a substantial portion of the volume
of the system, and the fluctuations are comparable to those in the critical
region. For these reasons, we dub this phase as the {\it subcritical liquid}.
The qualitative picture that we draw is in agreement with previous studies
based on similar effective models. We also clarify the discrepancy on the
behavior of the critical temperature versus $L$ found in different models.
