Paper Chromatography

A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Paper chromatography is a good way of separating coloured substances mixed together in a solution. During chromatography, the solvent soaks up the paper past the spot of the mixture and the different substances move upwards too. They move different distances depending on what kind of substance they are, what the solvent is and how far the solvent itself travels.
The method allows us to compare an unknown substance with a known one. If two substances travel the same distance in the same experiment, they are probably the same substance.

In this experiment, we will show how to separate the ink from different coloured markers using paper chromatography.

Method

1. Take a piece of chromatography paper. Draw a pencil line across it about 1 cm from one end and mark small crosses on the line.
2. Using coloured markers, place small spots of colour on the crosses.
3. Put water in the beaker to a depth of about 0.5 cm.
4. Hang the chromatography paper in the beaker so that it dips into the water but with the spots of colour above the water level.
5. Allow the water to rise up the paper until it is almost to the top.
6. Remove the paper, mark where the water reached (solvent front) and hang it up to dry.
7. When dry, measure the distances moved by the spots and the solvent front.

Questions

1. Why should the line at the bottom of the paper be drawn in pencil?
   
2. What would happen if the level of the solvent came above the level of the spots at the start?
   
3. Why should the solvent not be run off the top of the paper?

Solutions

• Show Solutions
  1. We use a pencil because it does not dissolve or smear when the solvent rises past the line.
  2. If the solvent started above the samples, the sample spots will simply dissolve and will not move up the paper.
  3. If the solvent ran off the top of the paper, we will not be able to measure the solvent front which is needed to calculate the R_f values.

How to use paper chromatography to separate soluble substances and identify an unknown compound?

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