1. Chromatography

2. Starter Match these key words with the correct definitions:
NON-AQUEOUS SOLUTION
SOLVENT
SOLUTION
AQUEOUS SOLUTION
SOLUTE

3. Starter Match the key words with the correct definitions:
SOLUTION
a solute dissolved in a solvent
SOLUTE
a substance that is dissolved in a solvent.
SOLVENT
a liquid in which chemicals dissolve to make a solution (e.g. water, ethanol)
AQUEOUS SOLUTION
solution where a solute is dissolved in water
NON-AQUEOUS SOLUTION
solution where the solute is dissolved in a solvent that isn’t water (e.g ethanol)

4. Learning Objectives Understand when chromatography is used
Describe the method of chromatography
Know how to carry out paper chromatography
Analyse chromatograms
Calculate Rf values

5. When is chromatography used?
Paper chromatography is a technique for separating components of a mixture.
Which kind of mixtures ?
Water soluble mixtures such as:
Colourings
agents
Dyes/paints
Inks

6. Simple Chromatography
Chromatography is an analytical technique that separates components in a mixture between a mobile phase and a stationary phase
The mobile phase moves, it can be a liquid or a gas.
The stationary phase doesn’t move. It can be a solid or a suspended liquid.

7. How it works Some substances are more soluble than others.
Substances that don’t dissolve, don’t move.
Substances that dissolve the most move the farthest.
solvent front
most soluble pigment
partly soluble pigment
origin
Insoluble pigment

8. How it works
With a solid stationary phase, the pigments are adsorbed onto the surface of the solid particles or fibres.
The relative forces of attraction between the stationary phase, the mobile phase and the pigment mean that separation happens
pigment with weakest forces of adsorption
Pigment with strong forces of adsorption
Insoluble pigment

9. How does it work? chromatogram
The water is drawn up by capillary action (sticky attractive force between water molecules and the paper).
When the solvent reaches the ink spots they are dissolved and carried with the water as it moves up the paper.
The most soluble ink travels the fastest, whilst the less water-soluble inks are left near the bottom of the paper.
Solvent
front
chromatogram
The colours separate out because they dissolve in the solvent (soluble)
Separated
dyes
Base line
(pencil line)

10. Analysing chromatograms
Start
Finish
How many colours in the red dye?
Which colours is the purple dye made out of?
Which dyes are made using yellow colour?

11. Analysing chromatograms
1) How many dyes were used in Black?
2) Are the yellows in brown and orange the same?
How do you know?

12. Inks i, ii, iii and iv were made from colours A, B, C and D
Try to work out which colours were mixed for each of the inks (i-iv)

13. Rf values
The ratio between the distance travelled by the pigment and the distance travelled by the solvent front is a constant, Rf.
(provided temperature, mobile phase and stationary phase are the same)
solvent front
most soluble pigment B
partly soluble pigment A
origin

14. Rf Values (retention factor)
Distance travelled by component
Distance travelled by mobile phase
Specific to each plate
Comparable
Each component on plate has unique Rf value

15. The answer is never greater than 1.
Calculating Rf values
A chemical can also be identified by its Rf (Retention Factor)
It is the ratio of the distance travelled by the sample (spots) to the distance travelled by the solvent (water).
The formula is:
Rf = distance moved by sample
distance moved by solvent
The answer is never greater than 1.

16. Calculate Rf for the three spots.
Calculating Rf values
Calculate Rf for the three spots.
Substance
Distance moved by sample (cm)
Distance moved by solvent front (cm) Rf
E120
7.2 10
0.72
E133
4.1
0.41
E124
9.5
0.95

17. Past Paper Questions

18. Past paper question

19. Mark scheme

20. Challenge question - 1
This question relates to the chromatogram shown in the earlier question. Refer back…

21. Mark scheme

22. Challenge question 2

23. Mark scheme – check your answer