Chromatography

• 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:

  Rf = distance moved by substance / distance moved by solvent 
• 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.