  We present a new particle based code with a multi-phase description of the
ISM implemented in order to follow the chemo-dynamical evolution of galaxies.
The multi-phase ISM consists of clouds (sticky particles) and diffuse gas
(SPH): Exchange of matter, energy and momentum is achieved by drag (due to ram
pressure) and condensation or evaporation. Based on time scales we show that in
Milky-Way-like galaxies the drag force is for molecular clouds only important,
if their relative velocities exceed 100 km/s. For the mass exchange we find
that clouds evaporate only if the temperature of the ambient gas is higher than
one million Kelvin. At lower temperatures condensation takes place at time
scales of the order of 1-10 Gyr.
