Sciences

Distillation: Separating a Mixture by Boiling Point

Distillation boils a mixture, cools the vapour back to a liquid and collects it. Simple recovers a solvent; fractional splits liquids with close boiling points.

Distillation

Distillation is a separation method in which a mixture is boiled and the vapour given off is cooled back into a liquid and collected apart from the rest.

Où les élèves le rencontrent: Grade 6 science, in the unit on separating mixtures, and again in Grade 10 chemistry where crude oil is split into fractions.

La réponse en bref
Distillation separates liquids by boiling point. The mixture is heated until one component boils, the vapour passes into a condenser and is cooled back into a liquid, and that liquid — the distillate — is collected. Simple distillation recovers a solvent from a solution; fractional distillation separates liquids that boil at similar temperatures.

Un exemple

Seawater → heat to 100 °C → condense the steam → pure water; the salt stays behind

Water boils at 100 °C and sodium chloride melts at 801 °C, so at the temperature of boiling water only one of them can leave as a vapour. The steam travels into the condenser, cools and drips into the receiving flask as pure water. The salt is left in the original flask as a dry residue. The thing to keep straight is which of the two you were asked for: the distillate is the water, not the salt.

The apparatus, and the three things marked wrong

Distillation diagrams lose more marks than the theory does, and always in the same places. The thermometer bulb belongs level with the side arm, not down in the liquid, because what matters is the temperature of the vapour leaving for the condenser. The condenser water goes in at the bottom end and out at the top, so the jacket fills completely and the coolest water meets the vapour last. And the apparatus must not be sealed at the end, or pressure builds up.

The rest is straightforward: a flask with a few anti-bumping granules to stop the liquid boiling in violent surges, gentle heat, and a receiving vessel for the distillate.

Simple against fractional

Simple distillation is enough when only one component will vaporise, or when the boiling points are far apart. Pure water from seawater is the standard example, and the Gulf states use the same principle at enormous scale to make drinking water.

Fractional distillation is for liquids that boil at close temperatures, where a single boil would carry over a mixture of vapours. A fractionating column packed with glass beads sits between the flask and the condenser. Vapour rises, condenses on a bead, is re-boiled by the vapour behind it, and rises a little further, so the mixture is effectively distilled many times over on the way up. The column is cooler at the top, so only the substance with the lowest boiling point reaches it. Ethanol at 78 °C comes over first and water at 100 °C follows.

What it is used for, and where it stops

The method scales from a test tube to a refinery, but it is not universal. Anything that decomposes when heated cannot be distilled, and two liquids with the same boiling point cannot be separated by it at all. Ethanol and water are a known awkward case: fractional distillation will not take ethanol past about 96 per cent, because at that composition the mixture boils as though it were a single substance.

Seawater into drinking water — simple distillation, used industrially across the Gulf
Ethanol from a fermented mixture — fractional, ethanol boiling at 78 °C
Liquid air into its gases — fractional, nitrogen boiling off at about −196 °C before oxygen at about −183 °C
Crude oil into fractions — refinery gases, petrol, kerosene, diesel, fuel oil and bitumen, each collected over its own range of boiling points

Questions fréquentes

What is the difference between simple and fractional distillation?

Simple distillation has no fractionating column and separates substances whose boiling points are far apart, such as water from dissolved salt. Fractional distillation adds a column that repeats the evaporation and condensation many times on the way up, which is what makes it possible to separate liquids boiling only a few degrees apart.

Why is the thermometer placed at the side arm?

Because it is there to measure the vapour, not the liquid. The reading tells you what is coming over into the condenser at that moment, so a steady reading at a known boiling point means the right fraction is being collected. A thermometer in the liquid measures the mixture that is being left behind.

Which way should the condenser water flow?

In at the lower end and out at the upper end. Filling against gravity keeps the outer jacket full rather than half empty, and it means the vapour meets progressively colder water as it travels, so it condenses fully before reaching the outlet. Reversed flow is a standard lost mark.

Can distillation separate salt from water?

Yes, and it gives you both. The water comes over as distillate and the salt is left as a dry residue in the flask. If the water is all you want, simple evaporation would waste it; if the salt is all you want, evaporating the solution is quicker and no condenser is needed.

Dernière mise à jour 21 août 2026