Chromatography is an analytical method used to separate a mixture of soluble substances ( dyes and pigments) or to test the purity of a mixture to identify them.
The pigment must be soluble in a liquid. This will allow the substances in the spot to be dissolved in the solvent and they will move up the paper with the solvent. This allows the different substances in the sample to separate out.
The spot of mixture must be above the level of liquid. The bottom of the paper should be dipped in solvent but not the spot must remain above the surface level of the liquid.
Otherwise, the spot will be dissolved in the solvent and won’t be separated.
Rf value is the ratio of distance travelled by the solvent divided by the distance travelled by the solvent. If two substances have the same Rf value, then they are likely the same. If the spot has substances of the same Rf value then it’s not a mixture, it is a pure substance.
Separation of substances using chromatography does not require the spot to be colored. If substances in the spot are colorless, the chromatogram can be sprayed with a chemical called a locating agent to show where the spots are.