• 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

• These values are used to?identify?the components of mixtures
• 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:

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?R_f?value is a ratio and therefore has no units

Using R_f?values to identify components of a mixture