What is distillation?

The physics of the separation

Distillation rests on the fact that, at a given temperature, the components of a mixture have different vapour pressures. When a mixture is heated, the component with the lower boiling point — the more volatile one — evaporates more readily, so the vapour above the liquid is enriched in it. Leading that vapour to a cooler surface, the condenser, removes heat and turns it back into a liquid. The condensate is therefore richer in the volatile component than the original mixture, while the residue left behind is richer in the higher-boiling component.

Simple and fractional distillation

Simple distillation involves a single evaporation–condensation step and works well when the components have widely separated boiling points or when one is essentially non-volatile.

When boiling points are close, fractional distillation is used. A packed or plated fractionating column sits between the flask and condenser; vapour repeatedly condenses and re-evaporates as it rises, each cycle enriching it further in the volatile component. This gives much sharper separation, as in the refining of crude oil into fractions.

Uses in research

Distillation is a fundamental purification and separation method in chemistry, used to purify solvents, isolate reaction products, and recover volatile compounds. Related variants extend its range: vacuum distillation lowers the boiling point to protect heat-sensitive compounds, while steam distillation isolates substances such as essential oils. It is a preparative complement to analytical separations like chromatography, and purity of the distillate is often verified by techniques such as gas chromatography.