Distillation Theory Part III: Vacuum Distillation
In the previous post, boiling was described as the temperature where the vapor pressure of the liquid is equal to atmospheric pressure. At this temperature, the liquid can freely migrate into the gas phase. The amount heat being applied to the liquid determines how quickly the entirety of the liquid becomes gas.
However, this is simply the boiling temperature at atmospheric pressure. Boiling can also be defined as the temperature where the vapor pressure of the liquid is equal to the pressure of the environment that the liquid is in. Frequently, chemists and processors place liquids in closed systems like reactors or flasks. Then the chemist can use a vacuum pump to remove air from the system and set a lower pressure.
When this is done, the boiling temperature of the liquid is lowered. The temperature required to have the vapor pressure equal the environment is much lower. In our water example from the last post, if a pressure of 17.54 mmHg is applied to a flask of water, the boiling temperature of the water will be 20°C. If the flask only contains pure water, the water will stay at 20°C until all the water is boiled out of the flask.
Scientists construct vapor pressure curves, like the graphic of this post. These curves are used to help select pressures for distillation. It isn’t always useful to run distillations at atmospheric pressure, because the temperatures required can damage sensitive molecules. Because of this, vacuum is applied and the distillation is performed much cooler.
If a chemist has a reagent that needs to be redistilled, he can simply place it in an apparatus, go to the vapor curve, select a boiling temperature, apply the corresponding pressure and simply add some heat and the distillation will occur. The liquid will boil, the vapors will rise to the condenser, where they will turn back into pure liquid. The liquid will run out of the condenser to the distillate flask and pure reagent will be collected.
In the next post, I will start to explain how to use distillation to purify mixtures of materials.
Part 3 in the theory of distillation series.