Fractional Distillation

Introduction

Simple distillation cannot cleanly separate liquids such as ethanol and water, because both evaporate together. Fractional distillation solves this by adding one extra piece of equipment: a tall fractionating column between the flask and the condenser. With it, a mixture of liquids with boiling points only a few tens of degrees apart can be separated, one liquid at a time. It is used in laboratories, in the drinks industry and in huge oil refineries.

Core explanation

The apparatus. The set-up is the same as for simple distillation, with one addition. A vertical fractionating column sits on top of the flask. It is a glass tube packed with glass beads, rings or a spiral, or with indentations in the glass, giving a large surface area. At the top of the column is the thermometer, with its bulb level with the side arm, then a condenser and a receiver. Anti-bumping granules and gentle heating are used as before.

How it works. The mixed vapour rises into the column. The column is hot at the bottom and cooler at the top. On the way up, vapour repeatedly condenses on the packing and re-evaporates. Each time, the vapour that moves on is richer in the more volatile liquid, and the liquid that runs back down is richer in the less volatile one. By the time the vapour reaches the top, it is almost pure lower-boiling liquid. (The next page looks at what happens inside the column in more detail.)

Reading the thermometer. For a mixture of ethanol (78 °C) and water (100 °C):

- The temperature at the top rises, then stays steady at about 78 °C . This plateau shows that nearly pure ethanol is distilling. It is collected in the first receiver. - When the ethanol has almost all gone, the temperature rises again. The distillate collected during the rise is a mixture and is kept separately. - The temperature then settles at 100 °C as water distils. A fresh receiver collects it.

Each portion collected over one temperature range is called a fraction . The liquid with the lowest boiling point always comes over first.

Slow and steady. Heating must be gentle. If the mixture is boiled too fast, vapour rushes through the column without enough time to condense and re-evaporate, and the separation becomes poor.

More than two liquids. The same method separates three or more miscible liquids, which distil in order of increasing boiling point.

Step-by-step reasoning

To collect pure fractions from a mixture:

1. Heat the mixture gently in a flask fitted with a fractionating column. 2. Watch the thermometer at the top of the column. 3. Collect the distillate while the temperature holds steady at the first boiling point. 4. Change receivers when the temperature starts to rise. 5. Collect the next fraction at the next steady temperature.

Visual explanation

Picture a graph of the thermometer reading against time. The line climbs steeply, then runs flat at 78 °C for several minutes, climbs again, and runs flat at 100 °C. Beside each flat section is a labelled flask: "ethanol" and "water".

Real-world analogy

A fractionating column is like a series of sieves stacked on top of each other. One sieve lets through a mixture that is only slightly sorted, but after passing through many sieves in a row, only the smallest pieces reach the top. Each condense-and-evaporate step is one "sieving".

Real-world example

Perfume and flavour manufacturers use fractional distillation to separate the many substances in plant oils, such as lavender or orange oil. Different fractions contain different scent compounds, which can then be blended to make a product with a consistent smell.

Why?

Why does the lower-boiling liquid come over first? At each stage in the column, the more volatile liquid is more likely to stay as vapour, while the less volatile liquid is more likely to condense and run back. Only the lower-boiling liquid can stay as vapour all the way to the top while it is still present.

Common misconception

"The fractionating column is just to make the apparatus taller." The column's packing provides a large surface on which vapour condenses and re-evaporates many times. Without it, the separation is little better than simple distillation.

Worked example

Question: A mixture of methanol (65 °C) and propan-1-ol (97 °C) is fractionally distilled. Describe the thermometer readings and state which fraction is collected first.

Reasoning: The lower-boiling liquid distils first, giving a plateau at its boiling point; then the temperature rises to the second plateau.

Answer: Steady at about 65 °C while methanol is collected first, then a rise, then steady at about 97 °C while propan-1-ol is collected.

Quick check

1. In fractional distillation, which liquid distils first? Answer: The liquid with the lowest boiling point.

Exam focus

Draw and label the fractionating column between the flask and the condenser. Explain that the thermometer shows a steady temperature while each pure liquid distils, and that the receiver is changed when the temperature rises. Compare with simple distillation: same principle, but the column allows close boiling points to be separated.

Advanced insight

The quality of a column is measured in theoretical plates : each plate is equivalent to one complete evaporation and condensation step. A short packed column in a school laboratory might give a few plates; industrial columns can have dozens. The closer the boiling points, the more plates are needed for a good separation.

Summary

Fractional distillation separates miscible liquids with close boiling points. A packed fractionating column between the flask and the condenser allows repeated condensation and evaporation, so the vapour reaching the top is nearly pure lower-boiling liquid. The thermometer shows a plateau for each liquid; fractions are collected in separate receivers in order of increasing boiling point. Gentle heating gives the best separation.

Practice questions

1. What extra piece of apparatus is used in fractional distillation compared with simple distillation? Answer: A fractionating column, packed with glass beads or rings, between the flask and the condenser. 2. What does a steady thermometer reading show during fractional distillation? Answer: That one nearly pure liquid is distilling, at its boiling point. 3. Why should the flask be heated gently? Answer: So that the vapour has time to condense and re-evaporate many times in the column; fast heating gives a poor separation. 4. When should the receiver be changed? Answer: When the temperature starts to rise above the first steady value, showing that the first liquid has almost all distilled.