Separating mixtures

The substances in a mixture are not joined, so they can be separated by physical methods. Each method uses a difference between the substances: the size of the pieces (sieving), magnetism (a magnet), whether they dissolve (filtering), or their boiling points (distillation). Chromatography separates the dyes in an ink.

Key facts

  • Sieving separates pieces of different sizes; a magnet pulls out iron and steel.
  • Filtration separates an insoluble solid from a liquid: the solid left in the filter paper is the residue; the liquid that passes through is the filtrate.
  • A solute dissolves in a solvent to make a solution. Evaporation removes the solvent and leaves the dissolved solid behind.
  • Crystallisation: heat the solution to evaporate some of the water, then leave it to cool so crystals form. It gives pure crystals without overheating the salt.
  • Simple distillation gets a solvent from a solution: the liquid boils, the vapour cools in the condenser and is collected as the distillate.
  • Fractional distillation separates liquids with different boiling points: the liquid with the lowest boiling point is collected first.
  • Paper chromatography separates dyes: each dye travels a different distance up the paper. A pure substance gives one spot and melts at one sharp temperature.

Worked examples

Example 1

How would you get pure, dry salt from a mixture of salt and sand?

  1. Add warm water and stir: the salt dissolves, the sand does not.
  2. Filter: the sand stays in the filter paper (the residue); salty water passes through (the filtrate).
  3. Heat the filtrate in an evaporating basin until crystals form, then leave it to dry.

Answer: add water and stir, filter off the sand, then evaporate the water from the filtrate

Example 2

How can pure water be obtained from sea water? Name the parts of the apparatus.

  1. Heat the sea water. The water boils at 100 °C; the salt stays in the flask.
  2. The steam passes the thermometer and into the condenser, where cold water cools it back to a liquid.
  3. The pure water drips into the beaker: the distillate.

Answer: simple distillation: boil the sea water, cool the steam in a condenser and collect the water

Example 3

The chromatogram shows a black ink X and three dyes. How many dyes are in X, and which are they?

  1. Ink X splits into 3 spots, so it contains 3 dyes: it is a mixture.
  2. Each spot of X is level with one of the dyes A, B and C.
  3. Dyes at the same height in the same solvent are the same dye, so X contains A, B and C.

Answer: 3 dyes: A, B and C