Interpreting Chromatograms (Edexcel IGCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
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 Rf value allows chemists to identify unknown substances because it can be compared with Rf values 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 components of a mixture
Exam Tip
For the Rf calculations, both distances are measured from the baseline.
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