Required practial 7

• Describe how pigments from a leaf of a plant can be

  isolated with paper chromatography

  1. Crush leaves with solvent to extract pigments

  2. Draw a pencil line on filter / chromatography paper, 1 cm above bottom

  3. Add a drop of extract to line (point of origin)

  4. Stand paper in boiling tube of (organic) solvent below point of origin

  5. Add lid and leave to run (solvent moves up, carrying dissolved pigments)

  6. Remove before solvent reaches top and mark solvent front with pencil
• Explain why the origin should be drawn in

  pencil rather than ink. (2)

  ● Ink is soluble in solvent

  ● So ink would mix with pigments / line would move
• Explain why the point of origin should be

  above the level of the solvent. (2)

  ● Pigments are soluble in solvent

  ● So would run off paper / spots dissolve into solvent
• Explain why a pigment may not move up the

  chromatography paper in one solvent. (1)

  ● May be soluble in one solvent but insoluble in another
• Describe how pigments can be identified

  ● Rf value = distance moved by spot / distance moved by solvent front

  ● Compare Rf value to published value
• Explain why the solvent front should be marked

  quickly once chromatography paper is

  removed. (1)

  ● Once solvent evaporates, solvent front not visible
• Explain why the centre of each pigment spot

  should be measured. (1)

  ● Standardises readings as pigment is spread out

  ● So allows comparisons to be made
• Explain why the obtained Rf values were similar,

  but not identical, to the published values. (1)

  ● Different solvent / paper / running conditions may

  affect Rf value
• Explain why Rf values are used and not the

  distances moved by pigment spots. (2)

  ● Solvent / pigment moves different distances

  ● Rf value is constant for same pigment / can be

  compared
• suggest and explain the advantage of having different coloured pigments in leaves

  so leaves can absorb different wavelengths of light for photosynthesis