Choosing methods

Different methods are used to separate different mixtures. The method chosen depends upon the properties of the components of the mixture.

Separation methodSuitable for separating:
Filtration An insoluble solid from a liquid
CrystallisationA soluble solid from a solution
Simple distillationA solvent from a solution
Fractional distillationOne or more liquids from a mixture of liquids
ChromatographyColoured soluble substances

Using chromatography to check purity

A chromatogram produced by paper chromatography or thin layer chromatography (TLC) can be used to distinguish between pure and impure substances:

  • a pure substance produces one spot on the chromatogram
  • an impure substance produces two or more spots

In this chromatogram, brown ink is made of a mixture of red, blue and yellow substances. This is because the coloured substances in the brown ink travel the same distances (and have the same Rf values) as the reference substances.

Results of a chromatogram of three pure substances and brown inkInterpreting the chromatogram for a brown ink

Gas chromatography

In gas chromatography (GC):

The stationary phase is held on an unreactive solid support such as silica beads. These are packed into a long, coiled ‘column’ inside a thermostatically-controlled oven. A detector measures the amount of each substance in a mixture as it leaves the column:

  • different substances travel at different speeds through the column
  • they leave at different times, called retention times

The GC machine plots a graph of detector reading against retention time. Three main pieces of information can be gathered from a gas chromatogram:

  • the number of compounds in the mixture - the number of peaks
  • the amount of each compound present - the height of each peak (higher = more)
A graph of time in minutes against strength of response for a gas chromatogram

This gas chromatogram shows that:

  • six different substances were in the mixture (there are six peaks)
  • A was present in the smallest amount (it has the smallest peak)
  • B and C were present in equal amounts
  • F was present in the greatest amount (it has the largest peak)