At the beginning of the distillation, the distillation pot should be between one-half and two-thirds full. If the pot is too full, the surface area is too small for rapid evaporation and the distillation proceeds very slowly. If the pot is not full enough, there will be a large holdup volume and loss of sample. Holdup volume is the amount of vapor in the flask and head along with the liquid required to wet the inner walls of the apparatus. A typical holdup volume is one or two milliliters, which can lead to significant product loss, especially in a small scale experiment.

If you fill a flask and find it either too full or not full enough, choose another size flask from your lab drawer. The following photos show improper and proper levels of liquid in a distilling flask.