Separating Mixtures

One of the key differences between mixtures and compounds is that mixtures can be separated by physical means. The substances in a mixture are not chemically joined, so you can pull them apart without any chemical reactions. Different separation techniques work for different kinds of mixture. Filtering works for solids in liquids. Distillation works for liquids dissolved in liquids. Evaporation works for solids dissolved in liquids. Magnets work for magnetic substances. Chromatography works for separating very similar liquids. Knowing the right technique for the right job is one of the most useful practical skills in chemistry.

  • Filtering separatesSolid from liquidOr two solids of different sizes
  • Distillation separatesLiquids of different boiling points
  • Evaporation separatesDissolved solid from liquidSalt from sea water
  • Magnetism separatesMagnetic from non-magneticIron from sand
  • Chromatography separatesSimilar liquidsInks, drugs, blood components
  • Method depends onMixture typeAnd what you want to keep

Filtering

Filtering is the simplest separation method. A filter is any material with tiny holes that let smaller particles through but trap bigger ones. Common examples:

  • A tea bag lets water and dissolved flavour through but traps the tea leaves.
  • A coffee filter works the same way.
  • A kitchen sieve separates pasta from water.
  • Laboratory filter paper in a funnel separates fine solids from liquids.
  • Your kidneys are biological filters that remove waste from your blood.

Filtering only works for solids in liquids, where the solid particles are bigger than the filter holes. It does not work for solutions (where the solute particles are too small) or for two dissolved substances.

Evaporation

Evaporation works for separating a dissolved solid from a liquid. Heat the mixture (or just leave it in the sun) until the liquid evaporates away, leaving the solid behind.

This is how sea salt has been harvested for thousands of years: pump sea water into shallow ponds, let the Sun and wind evaporate the water, and the salt is left as solid crystals. Modern industrial salt production still uses the same basic technique. Evaporation also separates sugar from sugar cane juice, makes maple syrup from sap, and creates many other concentrated products.

Distillation

Distillation works for separating two or more liquids with different boiling points. The mixture is heated; the liquid with the lower boiling point evaporates first; the steam is collected and cooled back into liquid in a separate container. Distillation is a more sophisticated technique than evaporation and requires special equipment.

Distillation is used to:

  • Make drinking water from sea water in desert countries.
  • Refine crude oil into petrol, diesel, kerosene and many other products. (Each component boils at a different temperature, so they separate naturally.)
  • Produce spirits like whisky, vodka and gin from fermented liquids.
  • Make pure water for medicines and lab experiments.

Magnets

If a mixture contains magnetic substances (like iron, nickel, cobalt and steel), a magnet can pull them out. This is used to:

  • Separate iron and steel from rubbish at recycling plants.
  • Remove metal fragments from food in factories.
  • Separate iron filings from sand in school experiments.

Most other common materials (aluminium, copper, plastic, paper, glass, food) are not magnetic and pass right through.

Chromatography

Chromatography is one of the most powerful separation techniques. The principle is simple: different substances move at different speeds when carried through another material. The most familiar version is paper chromatography:

  1. Put a small dot of ink on a piece of paper.
  2. Stand the paper in a shallow tray of water (or other solvent).
  3. The water rises up the paper by capillary action, carrying the ink with it.
  4. Different colours of ink (like the dyes in a black marker) move at different speeds, separating into distinct horizontal bands.

Modern chromatography is used for much more sophisticated jobs: testing for illegal drugs in blood, identifying chemicals in food, checking the purity of medicines, even analysing the chemistry of distant planets through telescopes.

Fact The black ink in a typical felt-tip pen actually contains a mixture of several different colours (usually red, blue, yellow and others) that combine to look black. You can prove this at home with paper chromatography. Put a small dot of black marker on the edge of a piece of kitchen paper and dip the edge in water. Watch as the water rises up the paper, separating the black ink into its original colours. Different brands of marker use different mixes.

Other separation techniques

  • Decanting: pouring off the clear liquid from a settled solid (like wine from sediment).
  • Centrifuging: spinning a mixture extremely fast to force heavier components to the outside. Used to separate blood into cells and plasma, or cream from milk.
  • Sieving: separating different-sized solids using a mesh.
  • Crystallisation: slowly cooling a hot saturated solution so the solute comes out as pure crystals.
  • Floating and sinking: separates materials of different densities. Used in mining to separate valuable ore from waste rock.
Did you know? Crude oil straight out of the ground is a complex mixture of hundreds of different hydrocarbon chemicals. Modern oil refineries separate it through fractional distillation: heating it in a tall column, where each component condenses at a different height according to its boiling point. The lightest fractions (petrol, naphtha) come off near the top; medium fractions (diesel, kerosene) in the middle; heavy fractions (lubricating oil, tar) at the bottom. A single barrel of crude oil produces dozens of useful products.
Deeper dive: how chromatography revealed the structure of DNA

One of the most famous uses of chromatography in scientific history was the discovery of the structure of DNA. Before scientists could work out how DNA was built, they needed to know exactly what it was made of.

In the 1940s, British chemist Erwin Chargaff used a new type of chromatography to separate and measure the four chemical bases in DNA samples from many different species. He noticed a strange pattern: in every DNA sample, the amount of adenine (A) almost exactly matched the amount of thymine (T), and the amount of guanine (G) almost exactly matched cytosine (C). This became known as Chargaff's rules.

The reason for the rule was completely mysterious until 1953, when Watson and Crick worked out the famous double helix structure of DNA. The "ladder" of DNA has rungs made of paired bases: A always pairs with T, and G always pairs with C. Of course they should be in equal amounts: every A on one strand was matched by a T on the other.

This is a great example of how a separation technique (chromatography) provided the experimental evidence needed for a major scientific discovery. Without Chargaff's careful measurements, Watson and Crick would not have had the clue they needed to figure out the helix structure. Chromatography techniques are still used today across biology, medicine and forensic science, separating substances that other methods cannot.

For more, see solutions and mixtures vs compounds.