Chromatography

(Paper) Chromatography is used to separate dissolved substances from each other, such as food colouring.

1. First draw a base line on the chromatography paper with a pencil. which is insoluble, so it will not dissolve in the solvent.

2. Then place a spot of the sample mixture on the line.

3. Next, pour solvent into a beaker and pour some solvent (such as water). Then, Use a glass rod to tape the chromatography paper and place it on the top of the beaker allowing the paper to touch the solvent.

Make sure the pencil line is higher than the solvent. Also remember, the solvent does not 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 will 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 used to determine if a substance is pure or impure. If a substance is pure, they will only produce one spot because they are only made of one substance. Impure substances will produce two or more spots.