Module 6: Section 3

Question 1

What is chromatography used for?

Answer 1

Used to separate components in a mixture.

Question 2

What are the 2 types of chromatoghraphy you need to know about?

Answer 2

1. Thin-layer chromatography (TLC)

2. Gas chromatography (GC)

Question 3

Describe the method for setting up a Thin-layer Chromatography.

Answer 3

1. Draw a pencil line near the bottom of the plate and put a spot of the mixture to be separated on the line.
2. Dip the bottom of the plate into a solvent (usually ethanol)
3. As the solvent spreads up the plate, the different substances in the mixture move with it, but different distances, so they separate out.
4. When the solvent’s nearly reached the top of the plate, take the plate out and mark the distance that the solvent has moved (solvent front) in pencil.

Question 4

Give the equation for calculating an Rf value.

Answer 4

Rf =

distance travelled by spot / distance travelled by solvent

Question 5

What do Rf values tell you?

Answer 5

Allow you to work out what was in the mixture, by calculating a Rf value for each spot and looking them up in a table of known values.

Question 6

What does the distance a spot travels up the plate depend on?
Why do some travel less distance than others?

Answer 6

How far an organic molecule travels up the plate depends on how strongly its attracted to the layer of solid on the surface of the plate.
This attraction is called adsorption.
A substance that is strongly attracted will move slowly, so wont travel as far.

Question 7

Describe the set up of Gas Chromatography.

Answer 7

1. The sample to be analysed is injected into a stream of gas, which carries it through a coiled tube coated with a viscous liquid (oil) of solid.
2. Components of the mixture dissolve in the oil or onto the solid, evaporate back into gas and then redissolve along the tube.
3. Time taken for the substances to pass through the coiled tube and reach the detector is called the retention time.