Physical Chemistry , 1st ed.

Figure 7.14 shows plots of composition of liquid and vapor phase versus

temperature. The curious thing about this plot is that the bubble point line and

the dew point line touch each other at one point, then separate again. At this

point, the composition of the liquid and the composition of the vapor in equi-

librium with the liquid have the exact same mole fraction. At this composition,

the system is acting as if it were a single, pure component. This composition

is called the azeotropic compositionof the solution, and the “pure component”

having this composition is called the azeotrope.In the case of Figure 7.14, since

the azeotrope has a minimum temperature, it is called the minimum-boiling

azeotrope.For example, H 2 O and ethanol have a minimum-boiling azeotrope

that boils at 78.2°C and is 96% ethanol and 4% water. (The normal boiling

point of pure ethanol is just slightly higher at 78.3°C.)

Figure 7.15 shows a temperature-composition phase diagram for a negative

deviation from Raoult’s law. Again, there is a point where the bubble and dew

point lines touch, in this case forming a maximum-boiling azeotrope.Since we

180 CHAPTER 7 Equilibria in Multiple-Component Systems

T(BP 2 )

T(BP 1 )

Temperature

0.5

x 1 , y 1

0.0 1.0

Composition of

minimum-boiling

azeotrope

Bubble point line

Dew point line

Tie line

BP of

minimum-boiling

azeotrope

Figure 7.14 Temperature-composition phase diagram for a nonideal solution showing a pos-

itive deviation from Raoult’s law. Notice the appearance of a point in which liquid and vapor have

the same composition.

T(BP 2 )

T(BP 1 )

Temperature

0.5

x 1 , y 1

0.0 1.0

Bubble point line

Dew point line

Tie line

BP of

maximum-boiling

azeotrope

Composition of

maximum-boiling

azeotrope

Figure 7.15 Temperature-composition phase diagram for a nonideal solution showing

negative deviation from Raoult’s law. The azeotrope is maximum-boiling, rather than minimum-

boiling as shown in Figure 7.14.