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
NONAQUEOUS
SOLUTION
solution where a solute is dissolved in water
solution where the solute is dissolved in a
solvent that isn’t water (e.g ethanol)
When is chromatography used?
Paper chromatography is a technique for separating
components of a mixture.
Which kind of mixtures ?
Water soluble mixtures such as:
Inks
Dyes/paints
Colourings
agents
The Method
How does it work?
• 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 watersoluble inks are left near the bottom of the paper.
chromatogram
Solvent
front
Separated
dyes
Base line
(pencil line)
Practical
Identifying the colours in food dyes
Which M&Ms are pure colours
Which are made from a variety of colours
Carry out an experiment to find out!!!!!!!!!
The Method
1. Draw a pencil line 2cms from the bottom edge of the chromatographic
paper.
2. Lightly mark 4-5 small crosses along the pencil line (not on the right and
left edges of the paper)
3. Draw spots on top of each cross using different colours
4. Write the colour underneath each coloured dot, using a pencil
5. Fold the paper into a cylinder and fasten with a paper clip
6. Add water into a beaker at the depth of ½ cm (make sure that the water
line is below the pencil line) and stand the folded paper into the beaker
7. Wait for coloured spots to separate up the paper and then remove from
water and leave to dry.
Analysing chromatograms
Start
Finish
1) How many colours in the red dye?
2) Which colours is the purple dye made out of?
3) Which dyes are made using yellow colour?
Analysing chromatograms
1) How many dyes were used in Black?
2) Are the yellows in brown and orange the same?
How do you know?
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:
The answer is
never greater
than 1.
Calculating Rf values
Calculate Rf for the
three spots.
Substance
E120
E133
E124
Distance moved
by sample
(cm)
7.2
4.1
9.5
Distance moved
by solvent
front (cm)
10
10
10
Rf
0.72
0.41
0.95