The Foundations of Chemistry

As a result, unlike species hold one another especially tightly in the liquid phase, so fewer
molecules escape to the vapor phase. The observed vapor pressure of each component is
thus less than ideally predicted. An acetone–chloroform solution and an ethanol–water
solution are two examples that show negative deviations from Raoult’s Law.

FRACTIONAL DISTILLATION

In Section 13-8 we described simpledistillation as a process in which a liquid solution can
be separated into volatile and nonvolatile components. But separation of volatile compo-
nents is not very efficient by this method. Consider the simple distillation of a liquid
solution consisting of two volatile components. If the temperature is slowly raised, the
solution begins to boil when the sum of the vapor pressures of the components reaches
the applied pressure on the surface of the solution. Both components exert vapor pres-
sures, so both are carried away as a vapor. The resulting distillate is richer than the original
liquid in the more volatile component (Example 14-6).
As a mixture of volatile liquids is distilled, the compositions of both the liquid and the
vapor, as well as the boiling point of the solution, change continuously. At constant pres-
sure,we can represent these quantities in a boiling point diagram,Figure 14-12. In such
a diagram the lower curve represents the boiling point of a liquid mixture with the indi-
cated composition. The upper curve represents the composition of the vaporin equilibrium

14-10

chloroform CHCl 3

ethanol CH 3 CH 2 OH

14-10 Fractional Distillation 561

Figure 14-12 A boiling point diagram for a solution of two volatile liquids, A and B. The
lower curve represents the boiling point of a liquid mixture with the indicated composition.
The upper curve represents the composition of the vaporin equilibrium with the boiling
liquid mixture at the indicated temperature. Pure liquid A boils at a lower temperature than
pure liquid B; hence, A is the more volatile liquid in this illustration. Suppose we begin with
an ideal equimolar mixture (XAXB0.5) of liquids A and B. The point Prepresents the
temperature at which this solution boils, T 1. The vapor that is present at this equilibrium is
indicated by point Q(XA
0.8). Condensation of that vapor at temperature T 2 gives a liquid
of the same composition (point R). At this point we have described one step of simple
distillation. The boiling liquid at point Ris in equilibrium with the vapor of composition
indicated by point S(XA 0.95), and so on.

XA

XB

XB = 0.5

XA = 0.5

0

1 0

1

Temperature

bp pure B

bp pure A

T 1

T 2

Vap

or

Liq

uid

Q

S

P

R

T

The applied pressure is usually

atmospheric pressure.