First of all, let's look at how distillation works. We are all generally familiar with how distilled water is produced. The water is heated, and the steam or water vapor conducted away in a tube. If the tube is looped downward and cooling is applied below the hump, the vapor is condensed and distilled water obtained. This is "simple" distillation- i.e., removing a volatile substance (water) from non-volatile substances (lime, impurities, etc.).

"Fractional" distillation is used to separate mixtures of two liquids with different boiling points, such as alcohol and water. Ethyl alcohol with 4 percent water boils at approximately 173° F, while water boils at 212° F. A mixture of the two liquids will boil at all temperatures between 173° and 212°, depending on the ratio of alcohol to water.

Consider a beaker or a glass jug filled partially with a mixture of alcohol and water at some temperature. The top of the container is closed except for a small hole, to which a balloon is attached to keep air out. Thus, the vessel is at atmospheric pressure, but the enclosure above the liquid level is essentially undisturbed by air currents circulating around the jug.

After a period of time, the amount of water vapor and amount of alcohol vapor contained in the gaseous mixture above the liquid in the container will reach a constant value, depending on the temperature and pressure. The liquid and vapor mixtures reach an "equilibrium," a condition under which there is no net change in the liquid/vapor ratio or in the alcohol/water ratio within either the liquid or vapor mixture. However, the ratio of alcohol to water in the vapor phase is generally greater than the ratio in the liquid phase, because alcohol is usually more volatile than water (see Figure 1). It is this characteristic of a liquid-versus-vapor state of a substance that permits us to distill off an increasing concentration of alcohol from the alcohol/water mixture.

By bringing about a controlled series of successive sequences re-evaporation, condensation, re-evaporation and re-condensation), each re-condensation from the previous vapor state achieves a higher alcohol concentration. This is because the alcohol in the vapor is at a higher concentration than was the concentration in the liquid mixture from which it was vaporized.