12.3 Chromatography
- Syllabus
- 0620–2026–2027
- Topic
- 12.3
- Level
- —
Learning objectives
12.3.1Paper chromatography is used to• Describe how paper chromatography is used to separate mixtures of soluble coloured substances, using a suitable solvent12.3.2Interpret simple chromatograms to• Interpret simple chromatograms to identify: (a) unknown substances by comparison with known substances (b) pure and impure substances12.3.3Paper chromatography is used to• Describe how paper chromatography is used to separate mixtures of soluble colourless substances, using a suitable solvent and a locating agent Knowledge of specific locating agents is not required12.3.4And use the equation for Rf: Rf =• State and use the equation for Rf: Rf = distance travelled by substance distance travelled by solvent
Separate coloured substances by paper chromatography
Paper chromatography separates a mixture of soluble coloured substances using a suitable solvent.
- Draw a pencil baseline near the bottom of the chromatography paper.
- Place a small spot of the mixture on the baseline and let it dry.
- Stand the paper in a suitable solvent with the solvent level below the baseline.
- Let the solvent rise through the paper.
- Remove the paper before the solvent reaches the top, mark the solvent front in pencil and let the chromatogram dry.
The solvent carries the soluble substances up the paper. Components that travel different distances form separate coloured spots.
Use pencil because graphite does not dissolve and move with the solvent. Keep the starting spots above the solvent level, or they may dissolve directly into the solvent reservoir.
Interpret identities and purity on a chromatogram
| Observation under the same conditions | Interpretation |
|---|---|
| unknown spot matches a known spot in position / Rf | the known substance may be present in the unknown |
| one spot from a sample | sample is pure by this chromatogram |
| two or more spots from a sample | sample is a mixture / impure |
| no known spot matches | that component is not identified by the available standards |
Compare unknowns and standards made with the same paper and solvent. A matching travel position or Rf value supports the identification.
Two different substances can sometimes have the same Rf value, so their spots may overlap and appear as one. A chromatogram may therefore fail to reveal every component.
A spot is evidence for a component, not an element count or a molecular formula. More than one spot shows a mixture; it does not mean the sample is one compound made from several elements.
Reveal colourless substances on a chromatogram
Colourless soluble substances can be separated by paper chromatography, but their spots must be revealed after separation.
- Place the colourless mixture on a pencil baseline.
- Develop the chromatogram in a suitable solvent with the baseline above the solvent.
- Remove the paper, mark the solvent front and let it dry.
- Apply a locating agent so the separated colourless substances form visible spots.
The solvent moves and separates the substances; the locating agent reveals their final positions. The locating agent is applied after the solvent has separated the mixture.
The locating agent does not perform the separation and is not the solvent. The syllabus does not require the name of a specific locating agent.
Calculate and use an Rf value
An Rf value compares how far a substance travels with how far the solvent front travels from the same baseline.
R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}
Measure both distances from the pencil baseline in the same direction. For a spot, measure to its centre; for the solvent, measure to the marked solvent front.
If a spot travels 13.0 cm and the solvent front travels 15.1 cm: Rf = 13.0 ÷ 15.1 = 0.86. The units cancel, so Rf has no unit.
Under the same experimental conditions, compare an unknown Rf with known values to help identify the substance.
The substance distance is the numerator and the solvent-front distance is the denominator. A spot cannot travel beyond the solvent front, so a valid Rf is less than 1.