Chromatography
Paper chromatography
- Paper chromatography separates a mixture of soluble substances.
- Put a spot of the mixture near the bottom, stand the paper in solvent; as the solvent rises, substances move different distances and separate.
- Coloured substances show directly; colourless ones need a locating agent.
Practice
Paper chromatography separates substances by:
As the solvent rises, each substance moves a different distance, so they separate.
The $R_f$ value
- The finished paper is a chromatogram. Use it to identify a substance, or test purity (a pure substance gives one spot).
$$R_{\text{f}} = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent}}$$
Practice
A pure substance on a chromatogram gives:
One spot means pure; several spots mean a mixture.
Practice
A spot moves 4 cm and the solvent moves 8 cm. What is the Rf value?
Rf = distance of substance ÷ distance of solvent = 4 ÷ 8 = 0.5.
You've got it
Key idea
- chromatography separates soluble substances by how far they travel up the paper
- colourless spots need a locating agent; a pure substance gives one spot
- $R_f$ = substance distance ÷ solvent distance