Chromatography

Question 1

What is chromatography used for?

Answer 1

Separating mixtures of soluble substances, often pigments of ink.

Question 2

How does chromatography work?

Answer 2

Drops of different samples are placed in a horizontal pencil-drawn line on a piece of filter paper. The paper is placed in a solvent, often water. The solvent dissolves the samples and carries them up the paper. More soluble solutions are carried up the paper faster, and the substances separate out.

Question 3

What is needed for chromatography?

Answer 3

Solvent, filter paper, samples, container.

Question 4

What are the two phases of chromatography?

Answer 4

The stationary phase (a substance that does not move) and the mobile phase (a substance that moves through the stationary phase -the solvent).

Question 5

Why does chromatography work?

Answer 5

Each soluble substance forms bonds with the two phases. Substances that form stronger attractive forces/bonds with the stationary phase don’t move as far/fast and stay near the bottom. Substances that form a stronger attractive force with the mobile phase move towards the top.

Question 6

What can a chromatogram be used for?

Answer 6

Distinguishing between pure and impure substances (pure substances produce only one spot).
Identifying a substance by comparing the pattern of spots with unknown substances.
Identifying substances using Rf values.

Question 7

How do you work out Rf values?

Answer 7

Rf= distance travelled by spot
————————————-
distance travelled by solvent

Question 8

What units do Rf values have?

Answer 8

They have no units.

They vary from 0 (spot stays on baseline) to 1 (spot travels with solvent front)

Question 9

What is the Rf value for a spot that travelled 4cm from the start line if the solvent front is 10cm away from the start line?

Answer 9

0.4

4cm / 10cm = 0.4