Interpreting Chromatograms

Question 1

What does an Rf value represent in paper chromatography?

Answer 1

The ratio between the distance travelled by the dissolved substance (solute) and the distance travelled by the solvent

Question 2

What is the equation to calculate the Rf value of a spot on a chromatogram?

Answer 2

Question 3

How can you use chromatography to identify whether a substance is pure?

Answer 3

A pure substance won’t be separated by chromatography and will instead move as one blob, whereas a mixture would give you multiple spots

Question 4

Describe how chromatography is used to identify substances in a mixture by comparison with known substances.

Answer 4

• Run a sample of a substance that you think is in the mixture alongside a sample of the mixture itself
• If the sample has the same Rf value as one of the spots from the mixture, they’re likely to be the same substance

Question 5

Which other separation method could you pair with paper chromatography to analyse the composition of an ink?

Answer 5

Simple distillation

Question 6

Describe how you can use simple distillation and paper chromatography to investigate the composition of inks.

Answer 6

• Use simple distillation to work out what solvent the ink contains - it allows you to evaporate the solvent from the ink and collect it
• Thermometer in the distillation set-up will read the boiling point of the solvent when it’s evaporating, helping you to determine what it is
• Next, carry out paper chromatography on a sample of the ink - this will separate the different dyes in the ink
• You can compare the Rf values of different spots on the chromatogram with reference values to work out what dyes are in the ink