Thermodynamics and Chemistry

CHAPTER 2 SYSTEMS AND THEIR PROPERTIES

2.2 PHASES ANDPHYSICALSTATES OFMATTER 32

p

T

b

b

tp

cp

solid

liquid

gas

supercritical

fluid

b

bb

A b

B C

D

Figure 2.2 Pressure–temperature phase diagram of a pure substance (schematic).

Point cp is the critical point, and point tp is the triple point. Each area is labeled

with the physical state that is stable under the pressure-temperature conditions that

fall within the area. A solid curve (coexistence curve) separating two areas is the lo-

cus of pressure-temperature conditions that allow the phases of these areas to coexist

at equilibrium. Path ABCD illustratescontinuity of states.

completely disappear, and the substance has been completely transferred to the other phase.
If both phases coexist in equilibrium with one another, and the temperature and pressure of
both phases remain equal and constant during the phase transition, the change is anequilib-
rium phase transition. For example, H 2 O at99:61C and 1 bar can undergo an equilibrium
phase transition from liquid to gas (vaporization) or from gas to liquid (condensation). Dur-
ing an equilibrium phase transition, there is a transfer of energy between the system and its
surroundings by means of heat or work.

2.2.3 Fluids

It is usual to classify a fluid as either aliquidor agas. The distinction is important for a
pure substance because the choice determines the treatment of the phase’s standard state
(see Sec.7.7). To complicate matters, a fluid at high pressure may be asupercritical fluid.
Sometimes aplasma(a highly ionized, electrically conducting medium) is considered a
separate kind of fluid state; it is the state found in the earth’s ionosphere and in stars.
In general, and provided the pressure is not high enough for supercritical phenomena
to exist—usually true of pressures below 25 bar except in the case of He or H 2 —we can
make the distinction between liquid and gas simply on the basis of density. Aliquidhas a
relatively high density that is insensitive to changes in temperature and pressure. Agas, on
the other hand, has a relatively low density that is sensitive to temperature and pressure and
that approaches zero as pressure is reduced at constant temperature.
This simple distinction between liquids and gases fails at high pressures, where liquid
and gas phases may have similar densities at the same temperature. Figure2.2shows how
we can classify stable fluid states of a pure substance in relation to a liquid–gas coexistence
curve and a critical point. If raising the temperature of a fluid at constant pressure causes
a phase transition to a second fluid phase, the original fluid was a liquid and the transition