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The
pressure-temperature phase diagram for water is used to illustrate the
concept of state functions and the possibility of going from the liquid
phase to the vapor phase (or the other way) without a phase change (a
single phase throughout the process) by circumnavigating the critical
point, which is the highest temperature and pressure where two distinct
phases exist (647 K, 22.1 MPa for water). The piston and cylinder represent
the log of the volume, so that the large differences in volume between gas
and liquid can be visualized. Fluid with higher density is shown darker. The
liquid, vapor, and supercritical regions are labeled, but no definite
boundaries exist between these regions, because the transitions are continuous
when going around the critical point; however, a phase change is observed
when crossing the phase boundary. You can drag the green dot on the plot
to change pressure and temperature; the volume (obtained from the Peng-Robinson
equation of state) is displayed. The dot can only be moved from 10^{-3} to
10^{3} MPa and from the triple point to 720 K, so that the volume changes can
be more easily displyaed.

These simulations require the Mathematica CDF player,
available for free from the Wolfram website.
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