How to Read a Fractional Distillation Graph

Look at the x-axis and the y-axis to see which units are listed on each. The y-axis will generally be temperature, while the x-axis will show the fraction of the mixture formed by one of the two components. This number is called the mole fraction.

Notice the two curved lines on the graph. The lower line represents the boiling point of the liquid for any given mole fraction value, while the upper line represents the composition of the vapor.

Recognize that when the liquid boils, you can find the mole fraction of the vapor by drawing a horizontal line from the "liquid" line at that temperature to the "vapor" line. That's because the vapor contains a greater portion of the more volatile substance than the liquid mixture. Condensation, by contrast, is represented by a vertical line, since the composition of the liquid formed by condensation is the same as the vapor.

Realize that fractional distillation is essentially repeating this process many times -- first boiling the liquid, then recondensing it, then reboiling it and so forth. Follow this process on your graph by drawing a horizontal line from the liquid curve to the vapor curve to represent boiling, then draw a vertical line back down to the liquid curve, then another horizontal line from this point back to the vapor curve and so on.

Notice that fractional distillation follows a kind of stair-step pattern along the graph; as you repeatedly boil and condense the liquid, you gradually move toward a liquid (or vapor) with a mole fraction of either 0 or 1, depending on whether the graph depicts the less or more volatile component.

In chemistry labs and in industry, chemists do not actually boil the liquid then recondense it, then reboil it and so on -- that would be extremely tedious. Rather, they use a "fractionating column", which contains many surfaces on which the vapor can condense. This accomplishes the same end result but is more convenient.