Chemistry - A Molecular Science

conditions. The energy required to convert a substance from the solid to the gas is known as the

heat of sublimation

,

HΔ

sub

. Figure 7.18 shows the pressure of the gas in

equilibrium with the solid as a function of temperature.

Solid

temperature

Vapor

pressure

Figure 7.18 Solid-vapor equilibrium

1

A

C B

Pressure (atm)

D o

solid

liquid gas

218

E

Temperature ( C)

0.0310.006

0

25 100

374

Figure 7.19 Phase diagram of water The temperature and pressure axes are not drawn to scale.

* All three phases are in equilibrium

at the triple point, so a mixture of

the solid and liquid can boil under t

hese conditions. In the case of

water, ice water boils at 0.01

oC and 0.006 atm, i.e., at its triple point.

1.0

A

B

C

Pressure (atm)

D

o

(^67) 5.1
solid
liquid
gas
73
-78 -56
E 25 31
Te m
perature
(C
)
Figure 7.20 Phase diagram of CO
(dry ice) 2
The temperature and pressure axes are not drawn to scale.
PHASE DIAGRAMS A phase diagram shows the phase of a substance as a function of temperature and pressure. It is simply a combination of diagrams like those shown in Figures 7.14, 7.15, and 7.18. A partial phase diagram of water is shown in Figure 7.19. The line separating the liquid and solid phases is similar to that shown in Fi
gure 7.14b and indicates the pressure dependence
of the melting point. The line separating the
liquid and the gas phases is the same one
displayed in Figure 7.15 and shows the vapor pressure of the liquid as a function of temperature (or the boiling point as a function of pressure). The line separating the solid and the gas is the same line shown in Figure 7.18 and shows the sublimation pressure as a function of temperature.
Consider the labeled points in the phase diagram for water shown in Figure 7.19. At
room conditions, which are typically taken to be 25
oC and 1 atm pressure (Point C), water
is a liquid with a vapor pressure of 0.031 atm. Point A is an equilibrium mixture of solid and liquid (a melt) at 1 atm, so the temperature at that point (0
oC) is the normal melting
point. Recall that increasing the pressure over a mixture always drives the mixture to the denser phase. Increasing the pressure at Point
A moves the liquid-solid mixture into the
liquid phase, so the liquid phase of water is de
nser than the solid phase. The liquid and gas
phases are in equilibrium at Point D. The pressure is 1 atm, so the temperature (100
oC) is
the normal boiling point of water. All three ph
ases are in equilibrium at Point B, so it is
the
triple point.

• The triple point of water occurs at 0.01
  oC and 0.006 atm.
  The critical
  point of water (Point E) occurs at 374.4
  oC and 217.7 atm.
  Some key features of the phase diagram of carbon dioxide are represented in Figure
  7.20. At room conditions (Point C), CO
  is a gas. Unlike water, liquid CO 2
  does not exist 2
  at 1 atm, so there is no normal boiling or melting point. Reducing the temperature at 1 atm results in deposition at -78
  oC (Point A), the temperature of ‘dry ice’. Carbon dioxide can
  be liquefied by increasing the pressure at 25
  oC to 67 atm (Point D). The triple point of
  carbon dioxide (Point B) occurs at 5.1 atm and -56
  oC. Note that the slope of the solid
  U
  liquid line indicates that solid CO
  is denser than liquid CO 2

. The critical point of CO 2

(^2)
(Point E) is at 31.1
oC and 73.0 atm. CO
is a supercritical fluid at points above and to the 2
right of Point E.
Chapter 7 States of Matter and Changes in State
© by
North
Carolina
State
University