During fractional distillation, why are glass beads placed into the column?
The question asks about a type of distillation called fractional distillation. Fractional distillation is a type of distillation where the components of a mixture are separated in sequence by first heating the mixture, then passing the vapors through a fractionating column that improves their purity, then condensing the purified vapors, and collecting them as liquid. We are also asked about the column of fractional distillation, so let’s have a look at the setup.
In the first step, the mixture is heated in a distillation flask, which may contain boiling chips to give a controlled boil and prevent the mixture from boiling vigorously and splashing or bumping. The more volatile components are converted to vapor and move up into the fractionating column. Here, in the fractionating column, which is step two, repeated cycles of condensation and reevaporation slowly separates out the components of the vapor, purifying them. The purified vapors pass through an adapter, pass the thermometer by which we can monitor which fractions are coming off, and move into the condenser where they are cooled and condensed into a liquid. This is step three. The liquid then moves through an adapter and is collected in a collection flask, step four.
That is the process of fractional distillation, but the question asks, “why are glass beads placed into the column?” It is talking about this fractionating column. The glass beads have a high surface area on which the heat from vapors can dissipate, causing vapors to cool and condense. But how does this high surface area cause separation in a multicomponent mixture? The mixture in the distillation flask may have multiple components. As the mixture boils, the vapors enter the column but predominantly the more volatile components or the more volatile substance or substances. And so the composition of the vapors entering the column might look like this. There is much more of the pink substance than at the bottom of the distillation flask.
Since it is cooler at the bottom of the fractionating column than in the distillation flask and since the glass beads provide a high surface area for heat energy from vapors to dissipate, the vapors begin to cool and condense. Liquid droplets composed mostly of the pink substance will form at the bottom of the fractionating column. More vapors enter the column and heat these liquid droplets and they evaporate again. This time, the vapors have an even higher proportion of the most volatile substance, in this case the pink substance. This cycle of condensation and reevaporation occurs over and over up the column. And the vapors become purer and purer towards the top of the column until finally the vapor leaving the column consists of one fraction only, and in this example one substance only, the pink substance.
However, if the pink component in the original mixture consisted of several substances, then the pink fraction coming off would also consist of several substances. They would be similar in boiling point. The question asked “During fractional distillation, why are glass beads placed into the column?” We’ve discussed fractional distillation in depth, but the simple answer is to increase the surface area available for condensing the vapor.