RP7 - Use of chromatography to investigate the pigments isolated from leaves of different plants

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 pigment
2) draw a pencil line on filter / chromatography paper , 1cm above bottom
3) Add a drop of extract to line ( point of origin )
4) stand paper in a boiling tube of solvent below point of origin
5) add lid and leave to run
6) remove before solvent reaches top and mark solvent front with pencil

Question 2

EQ : explain why the origin should be drawn in pencil rather than ink (2)

Answer 2

• ink is soluble in solvent
• so ink would mix with pigment

Question 3

EQ : explain why the point of origin should be above the level of the solvent (2)

Answer 3

• pigments are soluble in solvent
• so would run off paper / spots dissolve into solvent

Question 4

EQ : explain why a pigment may not move up the chromatography paper in one solvent (1)

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

EQ : explain why the solvent front should be marked quickly once chromatography paper is removed (1)

Answer 6

• once solvent evaporates , solvent front not visible

Question 7

EQ : explain why the centre of each pigment spot should be measured (1)

Answer 7

• standardises readings as pigment is spread out
• so allows comparisons to be made

Question 8

EQ : Explain why the obtained Rf values were similar , but not identical , to the published values (1)

Answer 8

• different solvent / paper / running conditions may affect Rf value

Question 9

EQ : explain why Rf values are used and not the distances moved by the pigment spots (2)

Answer 9

• solvent / pigment moves different distances
• Rf values is constant for same pigment / can be compared