Required practical 7

Question 1

Describe how pigments from a leaf of a plant can be isolated with paper chromatography.

Answer 1

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

Question 2

Explain why the origin should be drawn in pencil rather than ink.

Answer 2

● Ink is soluble in solvent
● So ink would mix with pigments / line would move

Question 3

Explain why the point of origin should be above the level of the solvent.

Answer 3

● Pigments are soluble in solvent
● So would run off paper / spots dissolve into solvent

Question 4

Explain why a pigment may not move up the chromatography paper in one solvent.

Answer 4

● May be soluble in one solvent but insoluble in another

Question 5

Describe how pigments can be identified

Answer 5

● Rf value = distance moved by spot / distance moved by solvent front
● Compare Rf value to published value

Question 6

Explain why the solvent front should be marked quickly once chromatography paper is removed.

Answer 6

● Once solvent evaporates, solvent front not visible

Question 7

Explain why the centre of each pigment spot should be measured.

Answer 7

● Standardises readings as pigment is spread out
● So allows comparisons to be made

Question 8

Explain why the obtained Rf values were similar, but not identical, to the published values.

Answer 8

● Different solvent / paper / running conditions may affect Rf value

Question 9

Explain why Rf values are used and not the distances moved by pigment spots.

Answer 9

● Solvent / pigment moves different distances
● Rf value is constant for same pigment / can be compared