Frequently asked in IGCSE Chemistry are questions on paper chromatography 😄
Paper chromatography is used to separate mixtures based on different solubility in a solvent.
How Paper Chromatography Works:
- Draw a base line on the chromatography paper with a pencil. You should not draw the line with ink because ink is soluble in solvents and will travel up the paper with the solvent.
- Place a drop of the mixture or dye on the base line.
- Lower the chromatography paper until the base line is slightly above the solvent. This is to prevent the dyes from dissolving and getting mixed in the solvent.
- The solvent will travel up the paper, taking the dye with it.
- Allow the solvent to reach the top of the chromatography paper.
- Since different substances have different solubility in the solvent, they will travel in different speeds; the one that has a higher solubility will travel farther in the same time.
- The same substance will travel the same distance on the chromatogram.
- In the diagram, dye 1, 2 and 3 are pure substances. The mixture is composed of Dye 1 and 3.
Calculating Retention Factor (Rf) Value:
Rf = (distance traveled by substance) / (distance traveled by solvent) = a / b
- Rf value of a particular substance is always the same under the same conditions.
- Knowing the Rf value can help to identify unknown substances.
Let's now take a look at some questions from IGCSE Chemistry exams!!
Example 1
Illegal drugs are sometimes used to affect the performance of racehorses. These drugs can be detected in horse urine using chromatography.
• A concentrated sample of urine from each horse is spotted onto the start line of a sheet of chromatography paper
• Known illegal drugs are also spotted onto the same paper
• Ethanol is used as the solvent
The chromatogram shows urine samples, A, B, C and D, and the two illegal drugs lasix and bute.
Explain which urine sample contains an illegal drug.
Sample C is composed of two substances. One of the substances traveled the same distance as bute, which means sample C contains bute.
Example 2
A student investigates some food colourings, each of which is made up of one or more dyes. She produces a chromatogram using the safe colourings red (SR), blue (SB) and green (SG) and food colourings red (FR), blue (FB) and green (FG). The diagram shows her chromatogram.
Explain which food colourings are defintely safe to use.
- FR only has components that traveled the same distance as components of SR. Therefore, it is safe to use.
- FB has components that are neither SR, SB, or SG. Therefore, we cannot be sure whether it is safe to use.
- FG only has components that are in line with components of SG. Therefore, it is safe to use.
Example 3
A student investigates the pigments found in some vegetables and fruit. She obtains some coloured vegetable and fruit extracts from carrots, tomatoes and sweet potatoes. She places a spot of each extract on chromatography paper, along with spots of the three pigments beta-carotene, chlorophyll and lycopene. The diagram shows the chromatogram at the end of the experiment.
Key to vegetable and fruit extracts and pigments
V1 = carrots, , V2 = tomatoes, V3 = sweet potatoes, P1 = beta-carotene, P2 = chlorophyll, P3 = lycopene
(a) Suggest a reason why there is a spot on the starting line in the chromatogram for sweet potatoes.
- Sweet potatoes has a substance that is insoluble to the solvent used in the paper chromatography.
- A: There is no substance in V1, V2 and V3 that is in line with chlorophyll, so A is correct.
- B: Beta-carotene is seen in all three carrots, tomatoes and sweet potatoes, so B is incorrect.
- C: The components of sweet potatoes are in line with beta-carotene and lycopene, so C is correct.
- D: Lycopene is present in all three carrots, tomatoes and sweet potatoes, so D is incorrect.
- E: The lowest spot on carrots and the middle spot on tomatoes are not in line with all three beta-carotene, chlorophyll and lycopene. It means that carrots and tomatoes contain other substances. Therefore, E is correct.