Separating Mixtures

In short: Mixtures are separated by physical methods that use a difference in the properties of their parts. Filtration removes an insoluble solid, evaporation and crystallisation recover a dissolved solid, distillation separates liquids by boiling point, and chromatography separates dissolved dyes.

The key idea

In a mixture, each substance keeps its own properties. So you can separate them by using a property that is different for each one:

• Particle size (sieving, filtering)
• Boiling point (distillation, evaporation)
• Density (decanting, separating funnel, centrifuge)
• Magnetism (magnetic separation)
• How well they dissolve or how they move through paper (chromatography)

All of these are physical methods. No new substances are made.

Simple methods

Filtration

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel.

• The solid that stays in the paper is the residue.
• The liquid that passes through is the filtrate.

Sand and water can be filtered. Salt water cannot, because the dissolved salt particles are tiny and pass straight through the paper.

Evaporation and crystallisation

To get a dissolved solid back out of a solution, remove the solvent.

• Evaporation: heat the solution until all the water has gone, leaving the solid. Quick, but crystals may be small or the solid may break down.
• Crystallisation: heat gently to remove some of the water, then leave it to cool. Crystals form slowly as the solution can no longer hold all the solute. This gives larger, purer crystals. Sea salt and sugar are made this way.

Distillation

Distillation separates a liquid from a solution, or two liquids with different boiling points. It keeps the liquid, unlike evaporation.

1. Heat the mixture. The substance with the lower boiling point evaporates first.
2. The vapour passes into a condenser, a tube cooled by cold water flowing around it.
3. The vapour condenses back into a liquid and drips into a beaker. This pure liquid is the distillate.

Distillation turns sea water into drinking water and separates alcohol from water.

Chromatography

Paper chromatography separates dissolved substances, such as the dyes in ink.

1. Draw a pencil line near the bottom of the paper (pencil, because ink would run).
2. Put a dot of the ink on the line.
3. Stand the paper in a small amount of solvent, below the line.
4. The solvent soaks up the paper and carries the dyes with it. Dyes that dissolve better and stick less to the paper travel further.

A pure substance gives one spot. A mixture gives several. Police use chromatography to match inks, and food scientists use it to check food colourings.

Quick summary

• Mixtures are separated by physical methods that use a difference in properties.
• Filtration: insoluble solid from a liquid. Residue stays, filtrate passes through.
• Evaporation and crystallisation: get a dissolved solid back.
• Distillation: get the liquid back, or separate liquids by boiling point.
• Separating funnel: two liquids that do not mix. Centrifuge: spin to separate by density.
• Chromatography: separate dissolved dyes. A pure substance gives one spot.
• Magnets pull out iron, nickel and cobalt.

Quiz