In this video, we will focus on fractional distillation.

Fractional Distillation Basics

Fractional distillation is used to separate a mixture of miscible liquids with different boiling points. Liquids with boiling points that are close together may be separated with this method, although better separation can be achieved if the difference in boiling points is more significant.

Miscible or Immiscible Liquids

Before we even proceed, you may be wondering, what is miscible? When we add different liquids together, some can mix well together (which we say they are miscible liquids), while some don’t (which we call them immiscible liquids). One very common example of immiscible liquids is the mixture of oil and water. When we allow the mixture to rest, the mixture eventually separates into distinct layers of liquids. However, if we add ethanol into water, they can actually mix well to form a solution. We do not see any layer in this mixture. These liquids are miscible, meaning, they can mix together completely to form a solution.

Fractional Distillation Set-up

Now, let’s take a look at the fractional distillation set up. (refer to video)

Here, we will separate a mixture of miscible liquids, which is, a solution of ethanol and water. We will heat up the mixture of liquids in the distillation flask. The purpose of the boiling chips in the flask is to ensure smooth boiling. Do note, the boiling point of ethanol is 78°C, while that of water is 100°C. Both liquids evaporate as the mixture is heated, but ethanol will boil before water.

As the vapour of both liquids rise to the fractionating column, they touch the glass beads in the column. There are many glass beads and they serve to provide large cool surface area for the vapour to condense on. In this case, at 78°C, vapour of water that touches the glass beads will condense back as liquid and drips back into the distillation flask. Only vapour of ethanol will be able to continue to rise up to the top of the fractionating column, enters the condenser, cools and condenses, and eventually be collected as a distillate in the receiver.

Hence, the purpose of the fractionating column, is really to separate the vapour of the different liquids in the mixture, so to ensure that, as much as possible, at any one time, only one vapour is able to enter the condenser.

Thermometer

The thermometer, which is placed at the top of the flask, is used to measure the temperature of the vapour that reaches the top of the column. When we are collecting ethanol, the thermometer shows a constant temperature of 78°C, which is the boiling point of ethanol.

The water, which has a higher boiling point, remains in the flask until almost all the ethanol has been distilled. When all the ethanol has distilled over, the temperature will rise to 100°C, which is the boiling point of water.

At this temperature, water distils over and be collected separately.

Condenser

Temperature in the condenser is much lower. The hot vapour cools down in the condenser, then condenses back into liquid. The pure liquid is collected as a distillate in the receiver. If the distillate to be collected is a volatile liquid, the receiver should be placed in an ice bath. This ensures the low temperature of distillate and allows it to remain as a liquid.

Do note that condenser should be sloping downwards to ensure that the pure solvent is able to flow downwards, into the receiver. Condenser has 2 tubes. Cold water flows in from the tube at the bottom, while flows out from the tube at the top. This ensures the whole condenser is filled up with cold water. This provides an efficient cooling system for the vapour.

Now you know how to separate a mixture of miscible liquids. What about immiscible liquids? Check out this video for the separation of immiscible liquids using separating funnel.

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