The Compleat Distiller

THE COMPLEAT DISTILLER 21
An illustration of vapor pressure
Fig. 2-2
The vapor pressure of a substance is the amount of pressure
escaped molecules can exert on the surrounding environment. A
simple experiment often performed in schools illustrates this.
Warning! Mercury is toxic; please do not attempt this
experiment yourself at home.
Take a long glass tube that is closed at one end, and fill it with
mercury. Upend it with the open end submerged in a beaker of
mercury. If the tube is long enough, an empty space will appear
above the mercury in the tube, and the mercury column will only
be about 760 mm (30 inches) high in the tube (tube A). This
empty space is a vacuum, and it's there because the weight of a
mercury column this high just balances the pressure of the
atmosphere. If we filled the tube with water, the liquid would be
about 9 meters (30 feet) high!
Now introduce a few drops of water into the bottom of the tube. They will float to the top of the
mercury and boil rapidly because they are now in a vacuum. At first, each droplet will boil off
completely and the level of the mercury will fall, because the water vapor created by the boiling
droplets exerts pressure on top of the mercury (tubes B and C).
Eventually, you will be left with a few drops of water floating quietly on top of the mercury. By this
time the mercury column will have lowered by about an inch or two. Adding more water will only
result in a layer of water on top of the mercury, but the open area above the liquid will not increase in
size (tubes D and E).
The lowering of the mercury column directly reflects the vapor pressure exerted by the water vapor.
Every substance can only exert a certain amount of vapor pressure at a given temperature. In a
confined space, there is a maximum concentration of molecules of a substance that may be present as
vapor. When this point is reached, the vapor is called saturated. In a saturated vapor, molecules are
condensing from vapor to liquid as rapidly as they are vaporizing. The pressure at which this occurs is
called the Saturated Vapor Pressure, and there is a characteristic saturated vapor pressure for every
compound.
If you increase the temperature of the apparatus, the water droplets will begin to boil again and the
mercury level will lower even more. At 100 º C (212 º F) the tube will contain only saturated water
vapor, because the vapor pressure of water at this temperature equals the pressure of the surrounding
atmosphere. This is the definition of the boiling point of a substance.
If you lower the surrounding pressure, you lower the boiling point. Increase the surrounding pressure,
as in a pressure cooker, and you raise the boiling point. This is very important to the process of
distillation. If you are interested in exploring this further, there is a comprehensive explanation in
Chapter 8, where we be show in detail how you can use this knowledge to calculate precisely the
concentration of ethanol you will get at each stage of distillation.
Boiling
Water droplets above the mercury column boiled at temperatures well below 100ºC, because the
pressure was much lower than normal. Evaporation from the surface of a liquid is a slow, stable
process that occurs at all temperatures. Boiling is a phenomenon that occurs when the vapor pressure
of the liquid exceeds the pressure inside the liquid and bubbles form. In boiling, evaporation happens
within the liquid and not just from its surface. The bubbles create new surfaces throughout the body of
the liquid.
Since the pressure within the liquid is slightly higher than that at the surface, because of the weight of
the liquid itself, the bubbles expand as they rise, which makes boiling more dramatic than evaporation,