Simple Distillation: Theory
Simple distillation is a technique that permits the separation of a solution into its respective components via differences in their boiling points. To understand how simple distillation works, let's first examine the boiling phenomenon.
Boiling (pure liquids)
As a liquid is heated, the kinetic energies (velocities) of molecules in the liquid phase increase. This additional energy permits more of the molecules to escape into the gas phase above the liquid and increases the pressure produced by the vapor. These results are usually displayed by a vapor pressure vs. temperature diagram (example at right).
The increase in vapor pressure also affects microscopic air bubbles within the liquid. As temperature increases, additional molecules force their way into the air bubbles from the liquid phase. The additional pressure produced increases the size of the air bubbles until the pressure inside is equal to the atmospheric pressure outside the bubble (Bubble formation at left)
Once the temperature has been increased to the point where the vapor pressure of the liquid is equal to the surrounding atmospheric pressure, the bubbles freely expand until they are large enough to break free and float to the surface of the liquid. This is known as boiling. The boiling point temperature is defined to be the temperature when the vapor pressure of the liquid is equal to the atmospheric pressure (1 atm for definition purposes). Always remember that your measured boiling point temperature depends on the current atmospheric pressure which is probably not 1 atm.
Steady or controlled boiling is observed when there are abundant microscopic air bubbles on the inside surfaces of the glassware to serve as starting points for boiling bubbles. However, most glassware has very smooth interior walls and consequently few air bubbles. Attempts to boil a liquid will lead to a few, BUT VERY LARGE, boiling bubbles that can splash the liquid sample out of the apparatus. This is known as bumping.
To reestablish smooth and controlled boiling we will add to the solution, before heating, a marble boiling chip. The rough surfaces of the boiling chip hide many microscopic bubbles that will initiate boiling bubbles. Remember to always use a fresh boiling chip to guarantee a fresh supply of air bubbles.