Edexcel IGCSE Chemistry

Revision Notes

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

Chromatography – Pure & Impure, IGCSE & GCSE Chemistry revision notes

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

Calculation

  • 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 mixture components, IGCSE & GCSE Chemistry revision notes

Using Rf values to identify components of a mixture

Exam Tip

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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