  In this paper, by considering a simple fluid model, we investigate the role
of phase transitions in the ISM on the galaxy scale gas dynamics. Cooling and
heating timescales in the ISM are typically shorter than typical galactic
rotation timescales, so the individual phases in the ISM can be assumed to be
in temperature equilibrium with the radiation field. Using this approximation
we can construct an equation of state which depends upon the average density
and mass fractions in the individual phases. Previous studies suggest that
there is an equilibrium phase fraction as a function of pressure. We
incorporate evolution towards this equilibrium state as a relaxation term with
a time to obtain equilibrium $\tau$. We derive a condition in terms of a
critical Mach number when one dimensional shocks should be continuous. For
small values of the relaxation time $\tau$ we show that the relaxation term
acts like a viscosity. We show with one dimensional simulations that increasing
$\tau$ causes shocks to become smoother. This work suggests that phase changes
can strongly effect the gas dynamics of the ISM across spiral arms and bars.
