Paper chromatography

Question 1

What is paper chromatography

Answer 1

Paper chromatography is used to separate a mixture of different dissolved substances which travel through a material. The dissolved substances are separated based on the solubility of the substance/ solvent.

Question 2

Give an example of a mixture of two or more dyes

Answer 2

Many inks and food colourings are mixtures of two or more dyes.
Paper chromatography is a very useful technique for separating these dyes

Question 3

The method by which paper chromatography is used (1)

Answer 3

• Absorbent paper is required for the process filter paper.
• A pencil line, known as a baseline or a datum line is drawn near the bottom of the paper and a small drop of mixture is placed on the line
• The bottom edge of the paper is placed in a solvent. The solvent must be able to dissolve the mixture, although some of the dyes will be more soluble than others

Question 4

The method by which paper chromatography is used (2)

Answer 4

• The solvent dissolves the mixture in the drop of ink and moves up the paper, carrying the dyes with it.
• The different dyes will travel up the paper at different rates.
• The dyes that are most soluble in the solvent will travel faster and therefore reach the highest level on the paper.
• The dyes that are less soluble will travel more slowly and not reach as far up the paper.
• Once the solvent has completed its movement up the paper is allowed to dry. There will be a pattern of colours on the paper, each one representing a part of the mixture.

Question 5

The retention factor (rf value) of a compound is equal to the…

Answer 5

distance travelled by the compound divided by the distance travelled by the solvent front ( both measured from the origin)

Question 6

The steps of chromatography (1)

Answer 6

• a drop of solution is dropped on the paper. The solvent evaporates. A spot of solute remains.
• The chromatography paper hangs from a glass rod. The paper must not touch the sides of the beaker.
• The spot of solute must be above the level of the solvent in the beaker.

Question 7

The steps of chromatography (2)

Answer 7

• The solvent travels up the paper to this level; its edge is called the solvent front
• Spots of different substances present in the solute separate as the substances travel through the paper at different speeds.
• The result is the chromatogram.