AQA GCSE Chemistry

Revision Notes

8.1.4 Required Practical: Investigating Chromatography

Required Practical 6: Investigating Chromatography

Objective:
Investigate how paper chromatography can be used to separate and identify a mixture of food colourings

Hypothesis:
Rf values can be used to identify the components of an unknown mixture by comparison with Rf values of known substances

Materials:

  • A 250 cm3 beaker
  • A wooden spill
  • A rectangle of chromatography paper
  • Four known food colourings labelled A–D
  • An unknown mixture of food colourings labelled U
  • Five glass capillary tubes
  • Paper clip
  • Ruler & pencil

Chromatography - Food Colourings, IGCSE & GCSE Chemistry revision notes

Diagram of the apparatus needed

Practical Tip:
The pencil line must never be below the level of the solvent as the samples will be washed away

Method:

  1. Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper
  2. Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line
  3. Use the fifth tube to put a small spot of the unknown mixture U on the line
  4. Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil
  5. Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots
  6. Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper
    should dip into the solvent
  7. Allow the solvent to travel undisturbed at least three quarters of the way up the paper
  8. Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line
  9. Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent
  10. For each of food colour A, B, C and D measure the distance in mm from the start line to the
    middle of the spot

Results:
Record your results in a suitable table

Required Practical 6:Investigating Chromatography Results, downloadable IGCSE & GCSE Chemistry revision notes

Evaluation:
The Rf values of food colours A, B, C and D should be compared to that for the unknown sample as well as a visual comparison being made

Conclusion:
The use of chromatography and Rvalues is a viable method of identifying unknown mixtures given reference material

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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