Fractional distillation is a very commonly used technique for separating mixtures of liquids.  It can be quite complicated in application, especially if azeotropes are present.  An azeotrope is simply a mixture which cannot be separated by distillation.  An example is ethanol/water.  No matter how many times you distill this, you are left with a 95% solution of ethanol; that last 5% of water cannot be removed unless somehow the mixture is altered (this is usually done, in this case, by adding some benzene; the resulting mixture can then be distilled to give 100% ethanol).

Be all that as it may, fractional distillation of ideal solutions (i.e., solutions which obey Raoult's law) can be fairly easily described at the level of this course.  (You're lucky here--usually this sort of pHun does not come about until you take later courses such as P. Chem.!)

The general idea here is that, as long as two liquids have different vapor pressures, distillation is possible.  Fractional distillation, in turn, can be made equivalent to doing several single distillations if an appropriate apparatus (column) is designed.

Consider a mixture of benzene and toluene.  This is, for all intents and purposes, an ideal solution (since the intermolecular forces between benzene and toluene molecules are almost identical).  The following graph shows the vapor pressures of the pure liquids and a 1:1 mixture of them as a function of temperature:

The mixture boils at 365.3K and you can determine the mole fractions of the two components from Raoult's law.  However, this is a bit tedious and, if you are doing repeated distillations, you will go rapidly crazy.

A better way of doing this is to use a phase diagram such as the one now shown.

The upper part shows the composition of vapor as one carries out a distillation.  The lower part shows the composition of liquid.  The white area in between represents a phase equilibrium.  That is, both phases a present at once.  The horizontal green lines are called tile lines.

Suppose that we start the distillation with a 50:50 mixture of liquid.  When we do this, a single distillation will then give vapor (which is then condensed) which is 71:29 (0.71 mole fraction benzene and 0.29 mole fraction toluene).  If you then take liquid of this composition (note the vertical green line to get to the next "starting point"), a second distillation will be a mixture of 86:14 in benzene and doing yet another distillation gives liquid which is 94:6 benzene/toluene.

A fractional distillation column which can accomplish this is said to have three theoretical plates.  That is, columns which enhance the number of effective distillations can be made and, quite often, separations resulting in essentially pure liquids can be obtained.  In other words, fractional distillation is simply the separation of mixtures into ever-increasingly purer fractions.

No problem was assigned for this at the time of writing; however, the concept is simple enough to allow it to "sneak into" an exam.  So, be prepared!