Why is the length of a fractionating column so important in a fractional distillation of ethanol?

Answer

Hi Yasmin,

In a distillation, the composition of the vapor is slightly richer in the most volatile component than the liquid. This is an equilibrium setting. In order to have the most volatile component almost pure, you therefore need a large number of equilibrium settings between vapor and liquid. Each equilibration occurs over a portion of the column’s length, so the longer your column, the more equilibrates and the pure the distillate.

An equilibrium setting is something that takes time. So if you heat slowly, vapor and liquid in the column will mix well, giving time to equilibrate. That is why you have to heat slowly.