Transformations at constant temperature (Delta T = 0)
An
isothermal process is a thermodynamic process in which the temperature of the
system remains constant. The heat transfer into or out of the system typically
must happen at such a slow rate that the thermal equilibrium is maintained. In a
phase diagram, an isothermal process is indicated by following a vertical line along
a constant temperature.
Therefore,
if the pressure and volume change, it is possible for a substance to change its
state of matter even while its temperature remains constant,