1.2.5Principles of Chemistry
Practical: Investigating Paper Chromatography — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 12 of 50
Part 1 of 5Aim and apparatus
Aim
- Aim To separate a mixture of food colourings and identify which known colourings it contains by comparing it with reference samples.
Apparatus
- Apparatus A 250 cm3 beaker, a rectangle of chromatography paper, a wooden splint, a paper clip, a pencil and ruler, glass capillary tubes,
- and solvent (water).
- Four known colourings labelled A to D and one unknown mixture labelled U.
How the method works
- How the method works The solvent moves up the paper and carries the dyes with it.
- Dyes that are more soluble in the solvent, or stick less to the paper, travel further, so a mixture separates into spots.
Part 2 of 5Method
1
Use a pencil and ruler to draw a horizontal start line about 2 cm from the bottom of the paper.
2
With a different clean capillary tube for each, place a tiny spot of colourings A, B, C and D and of the unknown mixture U on the line, and label each in pencil.
3
Keep every spot small, no more than 2 to 3 mm across, so the dyes do not smear.
4
Pour water into the beaker to a depth of less than 1 cm, so the surface is below the start line.
5
Clip the top of the paper to the splint and rest the splint across the beaker so the bottom edge dips into the water.
6
Leave the paper undisturbed until the solvent has travelled at least three quarters of the way up.
7
Remove the paper and at once mark the solvent front with a pencil line before it dries.
8
Let the paper dry, then measure the distance from the start line to the solvent front and from the start line to the centre of each spot.
Part 3 of 5Variables and safety
| Item | Detail |
|---|---|
| Independent variable | The sample being tested (colourings A to D and U) |
| Dependent variable | The distance each spot travels, and so its Rf value |
| Control variables | Same paper, same solvent, same start line height, same spot size, same time and temperature |
| Safety | Wear eye protection, wash hands after the experiment, and be careful if using a flammable solvent instead of water |
Part 4 of 5Processing the results
Rf = distance moved by spot ÷ distance moved by solvent
Calculating Rf
- Calculating Rf Rf = distance travelled by the spot ÷ distance travelled by the solvent front.
- Both distances are measured in the same units from the start line, so Rf has no unit and is always less than 1.
Identifying dyes
- Identifying dyes A spot in U that is level with a spot from a known colouring, and has the same Rf, shows that dye is present in the mixture.
Number of spots
- Number of spots A pure dye gives one spot, whereas a mixture gives two or more spots.
- Worked examplea spot moves 36 mm and the solvent front moves 60 mm, so Rf = 36 ÷ 60 = 0.60.
Part 5 of 5Common errors and fixes
| Error | Effect | Fix |
|---|---|---|
| Start line drawn in ink | Ink dyes run up the paper and confuse the chromatogram | Draw the line in pencil, which is insoluble |
| Large sample spots | Spots merge and smear, so Rf is inaccurate | Use small, concentrated spots |
| Solvent above the start line | Samples dissolve into the solvent and nothing separates | Keep the solvent level below the line |
| Solvent front reaches the top | The distance travelled cannot be measured | Remove the paper before the solvent reaches the top |
| Beaker uncovered | Solvent evaporates and the separation is affected | Cover the beaker with a lid or film |
Key terms
| Term | Meaning |
|---|---|
| Chromatography | A method of separating the substances in a mixture based on how far they are carried by a solvent through a medium such as paper. |
| Solvent front | The furthest point reached by the solvent on the chromatography paper. |
| Rf value | The distance moved by a substance divided by the distance moved by the solvent. |
| Chromatogram | The paper showing the separated spots after the experiment. |
| Mobile phase | The solvent that moves through the paper. |
| Stationary phase | The paper, which stays in place while the solvent moves over it. |
Avoid these mistakes
1
Drawing the start line in pen. The ink separates too, which spoils the results, so always use a pencil.
2
Starting with the solvent level above the start line, which washes the samples into the beaker.
3
Measuring to the edge or top of a spot instead of its centre, or forgetting to mark the solvent front before it dries.
4
Giving an Rf value with units or a value greater than 1. Rf is a ratio of two distances and is always between 0 and 1.
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