Page "Distillation" Paragraph 41

As the solution to be purified is heated, its vapors rise to the fractionating column.

As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material.

Here, the condensate continues to be heated by the rising hot vapors ; it vaporizes once more.

However, the composition of the fresh vapors are determined once again by Raoult's law.

Each vaporization-condensation cycle ( called a theoretical plate ) will yield a purer solution of the more volatile component.

In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column ; theoretical plate is thus a concept rather than an accurate description.