Separating mixtures

Every morning, many kitchens run a separation process: the coffee filter holds back the grounds and lets the coffee through. Salt from the sea, clean water from the sewage plant, and sorted waste all come from separation processes too. Time to understand the trick behind them.

The one trick behind every method

Separation always works the same way: you look for a property in which the substances in the mixture differ, and exploit exactly that difference. In filtering it is particle size: large particles like sand get caught in the filter, water runs through. In evaporating it is the boiling point: the water evaporates, the salt stays behind. In magnetic separation it is magnetism: iron jumps to the magnet, the rest does not.

Distillation and chromatography

Sometimes you want to keep not just the salt but also the water. Then you distil: the water evaporates, is collected, and condenses back into liquid H₂O in a cooled tube. This is also how alcohol is separated from water, because at normal air pressure alcohol already boils at 78 °C, while water boils at 100 °C. Chromatography separates even more finely: different dyes travel across filter paper at different speeds. That is how you can show that black felt-tip ink is really a mixture of several colours.

Separation in everyday life

In salt pans, also called saltworks, sea water is left to evaporate in shallow basins under the sun until only the salt is left behind. Sewage plants combine several methods at once: screens and filters hold back coarse material, and sludge settles to the bottom in settling tanks. And at the scrapyard, a huge magnet sorts the iron out of the metal pile. The same idea is at work everywhere: find the difference and use it.