• Chromatography can be used to separate mixtures and can give information to help identify substances. Chromatography involves a stationary phase and a mobile phase. Separation depends on the distribution of substances between the phases.
  • The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent can be expressed as its Rf value:

Different compounds have different Rf values in different solvents, which can be used to help identify the compounds. The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

Students should be able to:

  • explain how paper chromatography separates mixtures
  • suggest how chromatographic methods can be used for distinguishing pure substances from impure substances
  • interpret chromatograms and determine Rf values from chromatograms
  • provide answers to an appropriate number of significant figures.

WS 2.2 Plan experiments or devise procedures to make observations, produce or characterise a substance, test hypotheses, check data or explore phenomena.

WS 3.1 Presenting observations and other data using appropriate methods.

MS 1a - Recognise and use expressions in decimal form

MS 1c Use ratios, fractions and percentages

MS 1d Make estimates of the results of simple calculations

MS 2a Use an appropriate number of significant figures