A P-T diagram is the most common way to show
the phases of a substance. Figure 9 is the P-T diagram
for pure water. A P-T diagram can be constructed for any pure
substance. The line that separates
the solid and vapor phases is called the sublimation line.
The line that separates the solid
and liquid phases is called the fusion line. The line that
separates the liquid and vapor phases
is called the vaporization line.
The point where the three
lines meet is called the triple point.
The triple point is the only point at which all three phases
can exist in equilibrium. The point
where the vaporization line ends is called the critical
point. At temperatures and pressures greater
than those at the critical point, no substance can exist as a
liquid no matter how great pressure
is exerted upon it.
