5 · Distillation — Separation Techniques

The property being used

Definition Distillation is a process that separates substances with different boiling points, through boiling and condensation.

A mixture is boiled, and the vapour of the substance with the lowest boiling point enters the condenser first. That vapour then condenses when it meets a surface at a lower temperature. The condensed vapour is collected, leaving the other substances behind.

The liquid you collect is called the distillate.

Distilling salt solution

You keep—

Left in the flask—

Watch the thermometer stop at 100 °C and stay there. It reads the temperature of the vapour entering the condenser — the boiling point of whatever is coming across.

The three steps, in the words the exam wants 1. The salt solution in the distillation flask is heated until it boils.
2. Water changes into steam, which passes through the condenser. The condenser has cold water running through it continuously.
3. The steam cools and condenses into pure water, called the distillate. The salt remains in the distillation flask.

Boiling chips smoothen the boiling process — they stop the flask shaking too much.

Distillation in nature

The water cycle is distillation on a planetary scale: heating (evaporation) lifts water into the air, and cooling (condensation) turns it back into liquid, which returns to the ground as rain.

A solar still uses the same processes, and can be used to collect water where the supply is irregular. Water evaporates from the ground, condenses on the cool plastic sheet, runs down to the lowest point under the pebble, and drips into a glass as pure water.

Where it is used

Perfume making — one of the earliest uses. Petals are boiled in water and the vapour collected as the distillate is made into perfume.
Distilled water — water from natural sources contains minerals and impurities, which distillation removes. Used for drinking and in industries needing pure water.
Food flavourings — citrus oil and liquid extracts of herbs and spices.
Drinking water from seawater — desalination.
Oil refineries — on Pulau Bukom and Jurong Island, chemical engineers use distillation to produce a variety of fuels from crude oil.

Evaporation and distillation, side by side

Evaporation	Distillation
You collect the solid	You collect the liquid
The liquid escapes as vapour and is lost	The vapour is condensed and collected
Simple apparatus — a dish and a flame	Needs a condenser with cold water running through
Salt from seawater	Pure water from seawater

The property being used

Distilling salt solution

Distillation in nature

Where it is used

Check yourself