Concise Physical Chemistry

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