If you raise the pressure or lower the temperature of a gas or vapor, the molecules become more crowded. As they move closer and closer together, the mutual attraction of the molecules eventually will overcome the thermal energy keeping them in a gaseous state, and droplets of liquid will begin to form in the vapor. A vapor that has reached this point is called saturated. A very common example of this process is fog - a suspension of tiny water droplets in air, which is a mixture of gases and water vapor.
A substance that is normally liquid or solid still has a vapor pressure, and for any specific temperature, there is a specific pressure at which it begins to condense into liquid - the saturated vapor pressure. The saturated vapor pressure is the maximum pressure that a substance's can exert at a given temperature. The experiment with the mercury barometer in chapter 2 was an illustration of saturated vapor pressure. The important point to remember about saturated vapors is that they contain the maximum possible number of molecules possible at that temperature. Whenever you work with vapors, you need know if they are saturated or not. Their behavior depends upon the answer to that question.
This can easily be demonstrated with the same barometer experiment. Simply introduce a few drops of liquid, but not enough to saturate the space above the mercury, then slowly tilt the barometer so that the space gets smaller (slowly, to allow the heat generated by compression to escape and keep the temperature constant). At some point the liquid will start to condense out of the vapor, indicating that you have reached the saturated vapor pressure
In a distillation column, all the vapors are saturated throughout the system, and the temperature of the column at any point in the column is the boiling point of the mixture at that point. The boiling point of a substance is the point at which its saturated vapor pressure equals the pressure around it - in this case, atmospheric pressure. Just as saturated vapor forms above a boiling liquid, saturated vapor can condense back to a liquid at its boiling point This is the reason that reflux is so valuable.
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