Interpreting Chromatograms (Edexcel GCSE Chemistry: Combined Science)

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