In this experiment you will investigate two important parameters associated with columns that use bubble cap trays. The first parameter is the pressure drop that the vapor experiences as it passes through a bubble cap tray. This is clearly important in designing a distillation column, in order to determine the overall pressure drop through the column. The second parameter is known as the hydraulic gradient. Effectively the hydraulic gradient is the difference in the depth of the liquid across the tray; it provides the driving force for liquid to flow across the tray. If the hydraulic gradient is too large, then the depth of liquid near where it enters the tray is large, and the gas can’t bubble through the liquid easily. This leads to poor vapor-liquid contact and an inefficient column. If the hydraulic gradient is too small, dry spots can form on the tray, again leading to an inefficient column.