EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.2.4 Interpreting Chromatograms

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.2.4 Interpreting Chromatograms

Identifying Mixtures


  • Pure substances will produce only one spot on the chromatogram
  • If two or more substances are the same, they will produce identical chromatograms
  • If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
  • An impure substance therefore will produce a chromatogram with more than one spot





Diagram showing the analysis of a mixture and pure substances using chromatography



Rf Values


  • These values are used to identify the components of mixtures
  • The Rf value of a particular compound is always the same but it is dependent, however, on the solvent used
  • If the solvent is changed then the value changes
  • Calculating the Rvalue allows chemists to identify unknown substances because it can be compared with Rvalues of known substances under the same conditions
  • These values are known as reference values


  • The Retention factor is found using the following calculation:


Rf = distance travelled by substance ÷ distance travelled by solvent


  • The Rf value will always lie between 0 and 1; the closer it is to 1, the more soluble is that component in the solvent
  • The Rf value is a ratio and therefore has no units




Using Rf values to identify components of a mixture



Exam Tip

For the Rf calculations, both distances are measured from the baseline.