Solution vapor pressure has a direct bearing on fractional distillation, a procedure for
separating liquid components of a solution based on their different boiling points. Fractional
distillation is somewhat analogous to fractional crystallization. Suppose we want
to separate a binary system (a system with two components), say, benzene-toluene. Both benzene and toluene are relatively volatile, yet their boiling points are appreciably different
(80.1°C and 110.6°C, respectively). When we boil a solution containing these two
substances, the vapor formed is somewhat richer in the more volatile component, benzene.
If the vapor is condensed in a separate container and that liquid is boiled again, a
still higher concentration of benzene will be obtained in the vapor phase. By repeating
this process many times, it is possible to separate benzene completely from toluene.
In practice, chemists use an apparatus like that shown in Figure 12.9 to separate
volatile liquids. The round-bottomed fl ask containing the benzene-toluene solution is
fi tted with a long column packed with small glass beads. When the solution boils, the
vapor condenses on the beads in the lower portion of the column, and the liquid falls
back into the distilling fl ask. As time goes on, the beads gradually heat up, allowing
the vapor to move upward slowly. In essence, the packing material causes the benzenetoluene
mixture to be subjected continuously to numerous vaporization-condensation
steps. At each step the composition of the vapor in the column will be richer in the
more volatile, or lower boiling-point, component (in this case, benzene). The vapor
that rises to the top of the column is essentially pure benzene, which is then condensed
and collected in a receiving fl ask.
Fractional distillation is as important in industry as it is in the laboratory. The
petroleum industry employs fractional distillation on a large scale to separate the
components of crude oil. More will be said of this process in Chapter 24.