Chromatography

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

  1. A paper has a pencil line drawn horizontally. (Pencil is used instead of pen because pencil marks will not dissolve in the solvent)
  2. Place a small spot of the sample for the sample on the pencil line.
  3. The paper is then lowered into a solvent (such as water) – 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. As the solvent travels up the paper, it takes the ink particles that are dissolved in the solvent up the paper with it.
  5. As you can see in the diagram above, the substances in the sample spread out, causing a mixture of colours –
  6. The position where the solvent stops travelling is called the solvent front.
  7. 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 and some substances stick to the paper more 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.