with the boiling liquid mixture at the indicated temperature. The intercepts at the two
vertical axes show the boiling points of the two pure liquids. The distillation of the two
liquids is described in the legend to Figure 14-12.
From the boiling point diagram in Figure 14-12, we see that two or more volatile
liquids cannot be completely separated from each other by a single distillation step. The
vapor collected at any boiling temperature is always enriched in the more volatile compo-
nent (A); however, at any temperature the vapor still contains both components. A series
of simple distillations would provide distillates increasingly richer in the more volatile
component, but the repeated distillations would be very tedious.
Repeated distillations may be avoided by using fractional distillation.A fractionating
columnis inserted above the solution and attached to the condenser, as shown in Figure
14-13. The column is constructed so that it has a large surface area or is packed with
many small glass beads or another material with a large surface area. These provide surfaces
on which condensation can occur. Contact between the vapor and the packing favors
condensation of the less volatile component. The column is cooler at the top than at the
bottom. By the time the vapor reaches the top of the column, practically all of the less
volatile component has condensed and fallen back down the column. The more volatile
component goes into the condenser, where it is liquefied and delivered as a highly enriched
distillate into the collection flask. The longer the column or the greater the packing, the
more efficient is the separation.Distillation under vacuum lowers the
applied pressure. This allows boiling
at lower temperatures than under
atmospheric pressure. This technique
allows distillation of some substances
that would decompose at higher
temperatures.
562 CHAPTER 14: Solutions
Many fractions, such as gasoline,
kerosene, fuel oil, paraffin, and asphalt,
are separated from crude oil by
fractional distillation.
Figure 14-13 A fractional
distillation apparatus. The vapor
phase rising in the column is in
equilibrium with the liquid phase
that has condensed out and is
flowing slowly back down the
column.
Fractional distillation is used for
separations in many industrial
processes. In this Pennsylvania
plant, atmospheric air is liquefied
by cooling and compression and
then is separated by distillation in
towers. This plant produces more
than 1000 tons daily of gases from
air (nitrogen, oxygen, and argon).