Interpreting Chromatograms (Edexcel GCSE Combined Science: Chemistry): Revision Note

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

R_f Values

• These values are used to identify the components of mixtures

•  R_f stands for retention factor

• The R_f 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 R_f value allows chemists to identify unknown substances because it can be compared with R_f 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:

R_f = distance travelled by substance ÷ distance travelled by solvent

• The R_f value will always lie between 0 and 1; the closer it is to 1, the more soluble is that component in the solvent

• The R_f value is a ratio and therefore has no units

Using R_f values to identify components of a mixture