A level practical 7-9

Describe how pigments from a leaf can be isolated

1. Crush leaves with solvent to extract pigments

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

3. Add drop of extract to line

4. Stand paper in boiling tube of solvent -below pencil line

5. Add lid and leave to run

6. Remove before solvent reaches the top,mark solvent front with pencil

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

Ink soluble in solvent

So ink would mix with pigment

Explain why the pigment spots should be above the level of solvent

Pigments soluble

Dissolve into solvent

Explain why a pigment may not move up chromatography paper

May be insoluble in that solvent

Describe how pigment can be identified

Calculating rf value-distance spot/solvent

Compare to published vakue

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

solvent evaporates and is not visible

Explain why the centre of each pigment spot should be measured

Standardise as pigment is spread out

Allows comparison

Explain why rf values not identical to published

Different solvent

Paper may affect rf

Explain why rf values are used and not distance moved by pigment spots

Solvent/pigment moves different distances

Rf value constant for same pigment so can be compared