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Distillation — Chemistry, 14–17

Separating liquids by using different boiling points. Chemistry, 14–17 years.

Boil, then condense

Distillation separates substances by giving one component a way to evaporate and then turning that vapour back into liquid. A liquid with the lower boiling point usually enters the vapour first, so the collected liquid becomes richer in that component.

The problem of mixed liquids

Filtering works when a solid is trapped by a barrier, but dissolved liquids pass straight through. Distillation solves a different problem: it uses the unequal boiling points of liquids, which became especially useful for making clean water, alcohol and fuels from mixtures.

Separating ethanol and water

Ethanol boils at about 78 °C, while water boils at 100 °C. Heat a mixture carefully: vapour forms richer in ethanol, travels through the cooler condenser and becomes liquid again. Collecting that distillate gives more concentrated ethanol, though one pass does not make it perfectly pure.

It is not a magic filter

A common guess is that the liquid with the lower boiling point comes out alone, because it evaporates first. In a real mixture, both liquids contribute to the vapour, especially when their boiling points are close, so distillation usually enriches a component rather than separating it in one perfect step.

From seawater to medicines

Desalination plants can distil seawater, leaving dissolved salts behind, although the heating costs are high. Refineries also distil crude oil into fractions, and laboratories use distillation to purify solvents; each use depends on boiling behaviour, not on a substance being “dirty”.

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