c02 JWBS043-Rogers September 13, 2010 11:23 Printer Name: Yet to Come
THE CRITICAL TEMPERATURE 25
1
2
3
p
V
4
gas
liquid equilibrium vapor
FIGURE 2.5 Conversion of a liquid to its vapor without boiling (1–4).
gas, but it is usually applied to a gas in equilibrium with its liquid form. When a liquid
is in equilibrium with its vapor, heat can be applied with no change in temperature
but with conversion of some or all of the liquid to its vapor. When spheroids of vapor
rise from the bottom of a heated liquid to the top, we say that the liquid boils. It is
sometimes said that “no gas can be liquefied above the critical point.” This is true,
but it is a little misleading becausethere is no distinctionbetween liquid and gas
above the critical isotherm. Above the critical isotherm, the system is asupercritical
fluid.
The segment of the critical isotherm forming the upper boundary of the liquid
region is particularly interesting. The system passes from liquid to gas (or back again)
with no discontinuity. That is, it goes from liquid to gas but it does not boil. Points
just below the isotherm represent liquids of low density. Those above it represent
gases of high density. On the isotherm, the liquid and gaseous states become one and
the same.
To get a better feeling for the meaning of the critical isotherm, let us heat a
subcritical liquid (1) to one of its supercritical isotherms (2), expand it (3), and cool
it to its original temperature (4) as in Fig. 2.5. At the end of the process, the liquid
has been transformed to a state that is clearly in the gaseous region, but there is no
discernible phase change (boiling or vaporization→gas) during this process.
2.4.1 Subcritical Fluids
A subcritical curve in Fig. 2.5 has three real roots, predicting three different vol-
umes for the same fluid belowTc. This seems absurd. How can a gas have three
different volumes at the same time? The answer is that the term “fluid,” meaning
that which flows, is more general than “gas.” The term fluid includes both liquids
and gases. The volume of a subcritical liquid is small and is given by the leftmost
intersection (root) of the subcritical isotherm with the horizontal. The volume of
the vapor is large and is given by the rightmost intersection (root). The middle