Chromatography is a technique used to separate dissolved substances (e.g. food colouring) and analyse them. This is easier when the dissolved substances are different colours.

1. First, draw a baseline on the chromatography paper using a pencil, which is insoluble and will not dissolve in the solvent.

2. Next, place a spot of the sample mixture on the line.

3. Pour some solvent (such as water) into a beaker. Then, place a glass rod on top of the beaker and tape the paper to it, without allowing the paper to touch the solvent.

  • Make sure the pencil line sits above the level of the solvent. Also, remember that the solvent doesn’t have to be water; for example, it can be ethanol.

4. The solvent slowly travels upwards, taking with it a few soluble pigments from the sample mixture.

  • As you can see in the diagram above, the substances in the sample spread out, which forms different spots along the paper.

5. Remove the paper from the beaker before the solvent reaches the top.

  • The position where the solvent stops travelling is called the solvent front.

The paper is now called a chromatogram.

The substances in the sample spread out because some colours move faster than others. The substances in the sample travel up the paper at different speeds because some substances are more soluble than others.

Paper chromatography is also a useful technique for determining the purity of a substance. If a substance is pure, it will only produce one spot because it is only made of one substance. Impure substances will produce two or more spots.

Interpreting a Chromatogram

We can use a paper chromatogram to distinguish between pure and impure substances.

  • Pure substances only produce one spot on the chromatogram
  • Impure substances will separate, producing two or more spots

Substances that are the same will produce identical chromatograms. They will produce the same amount of spots, those spots will travel the same distances up the paper and the colours will match.

The brown ink is a mixture of red, blue and yellow inks. Therefore during chromatography, the colours separate. The inks that make up the brown ink travel the same distance up the paper as the reference inks.

For example, the red spot from the brown ink travelled the same distance up the paper as the red ink, which we used as a reference.