Distillation and fractional distillation

Two liquids that mix

Water and acetone mix together completely. We cannot use a separating funnel, because there are no layers. We cannot use filtration or evaporation to keep both. But the two liquids have different boiling points. This difference lets us separate them by distillation.

Simple distillation

Look at the apparatus in the picture.

1. The mixture is heated in a flask.
2. The liquid with the lower boiling point boils first, and its vapour rises.
3. The vapour goes into a condenser, a tube that is cooled by cold water around it.
4. The vapour condenses into a liquid, and the pure liquid drips into a receiving flask. This liquid is the distillate.
5. The liquid with the higher boiling point stays in the flask.

For a mixture of acetone (boiling point $329\ \text{K}$) and water ($373\ \text{K}$), the acetone distils first.

When can we use simple distillation? When the boiling points of the two liquids differ by more than $25\ \text{K}$. The thermometer helps us to keep the temperature at the boiling point of the first liquid.

Simple distillation is also used to get pure water from sea water, and the salt stays in the flask.

You should find that acetone and water give a sharp step, but two liquids only $10\ \text{K}$ apart give a gentle slope with no flat part.

Fractional distillation

When the boiling points are close, a simple distillation gives a distillate that is still a mixture. We use fractional distillation. A fractionating column is placed between the flask and the condenser. The column has rods or beads in it, and the vapour condenses and evaporates again many times as it goes up. At each step, the vapour has more of the liquid with the lower boiling point. At the top, the vapour is almost pure.

Separating the gases of air

Air is a homogeneous mixture of gases: about $78\ \%$ nitrogen, $21\ \%$ oxygen, and about $1\ \%$ of other gases such as argon. To separate them:

1. Air is compressed and cooled, and the water vapour and carbon dioxide are removed.
2. The air is made liquid by more compression and cooling.
3. The liquid air is warmed slowly in a fractionating column.
4. The gases boil off one by one, each at its own boiling point: nitrogen at $77\ \text{K}$, argon at $87\ \text{K}$, and oxygen at $90\ \text{K}$.

The boiling points are close (only $13\ \text{K}$ between nitrogen and oxygen), so fractional distillation is needed.

Other uses

• Crude oil is separated into petrol, diesel, kerosene and other fuels by fractional distillation.
• Alcohol is separated from the liquid made by fermentation.

Practise with new numbers

Try it in Python

This program chooses simple or fractional distillation from the boiling points.

Remember these

• Distillation separates two liquids that mix, by their boiling points.
• Simple distillation: the boiling points differ by more than $25\ \text{K}$. Fractional distillation: they are closer.
• Air is separated into nitrogen, argon and oxygen, and crude oil into its fuels, by fractional distillation.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Methanol boils at $338\ \text{K}$ and water at $373\ \text{K}$. Which distils first from their mixture?

2. Two liquids that mix boil at $360\ \text{K}$ and $376\ \text{K}$. Which kind of distillation is needed?

3. What does the condenser do in a distillation?

4. Why must air be made liquid before its gases can be separated by distillation?