Chromatography: Required practical activity 12: investigate how paper chromatography can be used to separate and tell the difference between coloured substances. Students should calculate R_f values.

The idea

Paper chromatography separates coloured substances because they travel at different rates through the paper as the solvent moves. Draw a pencil start line on chromatography paper, place small spots of each coloured substance on it, and lower the paper into a solvent so the solvent level is below the start line. Allow the solvent to rise, then mark the solvent front in pencil before it dries. Each coloured component forms a spot at a characteristic distance. Calculate R_f using R_f = distance travelled by the substance ÷ distance travelled by the solvent front. Compare R_f values of unknown spots with those of known substances to identify them; matching values suggest the same substance. A pure coloured substance usually gives one spot, while a mixture gives more than one spot.

What earns the marks

• Describe setting up paper chromatography with a pencil start line, small spots of coloured substances, and a solvent below the start line.
• Explain that components separate because they have different solubilities in the solvent and different attractions to the paper, so they move different distances.
• Measure the distance travelled by each coloured spot from the start line to the centre of the spot, and measure the distance travelled by the solvent front from the start line.
• Calculate R_f values using R_f = distance travelled by the substance ÷ distance travelled by the solvent front, and use the values to compare or identify substances.
• Interpret chromatograms: a single spot indicates a pure substance, while multiple spots indicate a mixture, and matching R_f values suggest the same substance.

Common mistakes and how to put them right

• Using a pen instead of a pencil for the start line or solvent front.

  Put it right by using pencil because pen ink would dissolve and run in the solvent.

• Placing the solvent level above the start line.

  Put it right by keeping the solvent below the start line so the spots do not wash into the solvent reservoir.

• Measuring the spot distance to its top edge rather than its centre.

  Put it right by measuring from the start line to the centre of the spot for a consistent R_f value.

Exam tips

• State clearly that the solvent front must be marked before it evaporates, because the R_f calculation depends on that measurement.
• Show the R_f equation and substitute the measured distances with units before calculating, so your method is clear.
• When identifying an unknown substance, compare its R_f value with the R_f values of known substances run on the same chromatogram under the same conditions.