An illustration of vapor pressure

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 °C (212 °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.

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