BRIDGE
Remember from Chapter 7 of MCAT General Chemistry Review that liquids boil when their
vapor pressure equals ambient pressure. In vacuum distillation, we lower the ambient
pressure so that the liquid can boil at lower temperatures.FRACTIONAL DISTILLATION
To separate two liquids with similar boiling points (less than 25°C apart), we use fractional
distillation. In this technique, a fractionation column connects the distillation flask to the
condenser, as shown in Figure 12.4. A fractionation column is a column in which the surface area is
increased by the inclusion of inert objects like glass beads or steel wool. As the vapor rises up the
column, it condenses on these surfaces and refluxes back down until rising heat causes it to
evaporate again, only to condense again higher in the column. Each time the condensate
evaporates, the vapor consists of a higher proportion of the compound with the lower boiling point.
By the time the top of the column is reached, only the desired product drips down to the receiving
flask.