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Finally, consider an initial pressure of
the ice of 0.08854 psia. Again, as a result of heat
transfer,the
temperature will increase until it reaches 32°F. At this
point, however, further heat transfer may
result in some of the ice becoming vapor and some becoming
liquid because it is possible to
have the three phases in equilibrium. This is called the triple
point, defined as the state in which
all three phases may be present in equilibrium. Figure
8 is a pressure-temperature diagram for water that shows
how the solid, liquid, and vapor phases
may exist together in equilibrium. Along the sublimation
line, the solid and vapor phases are
in equilibrium, along the fusion line, the solid and
liquid phases are in equilibrium; and along the
vaporization line, the liquid and vapor phases are in
equilibrium. The only point at which all
three phases may exist in equilibrium is the triple point.
The temperature and pressure for the
tri point
because there is no distinct change from the liquid phase
to the vapor phase above the critical
point.
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