Introduction
Distillation is a technique widely used in organic chemistry for separating compounds based on differences in their boiling points. Many organic compounds are volatile; that is, they have relatively high vapor pressures and low boiling points. During distillation, such volatile compounds are heated to boiling in one container, called the pot. The vapors produced are then cooled and condensed by passing them through a water-cooled condenser, and collected in a separate container, called the receiver. This technique can be used to remove a volatile solvent from a non-volatile product; or to separate two or more volatile products that have sufficiently different boiling points.
When a liquid is placed in a closed container, some of the molecules evaporate into any unoccupied space in the container. Evaporation, which occurs at temperatures below the boiling point of a compound, involves the transition from liquid to vapor of only those molecules at the liquid surface. Evaporation continues until an equilibrium is reached between molecules entering and leaving the liquid and vapor states. The pressure exerted by these gaseous molecules on the walls of the container is the equilibrium vapor pressure. The magnitude of this vapor pressure depends on the physical characteristics of the compound and increases as temperature increases.
If the liquid is heated to its boiling point, a quite different phenomenon occurs. The boiling point is the temperature at which the vapor pressure of the liquid is equal to the external pressure applied to the surface of the liquid. This external pressure is commonly atmospheric pressure. At the boiling point, bubbles of vapor are produced throughout the liquid, and the vapor pressure inside the bubbles is sufficiently high and allow them to grow in size. The escape of these bubbles results in the characteristic chaotic motion of the liquid identified as boiling.