A student uses paper chromatography to find which food colourings are in the coating of a sweet. The student puts a spot of the sweet colouring and spots of four known food colourings, E, F, G and H, on a pencil baseline. The paper is placed in a beaker containing a shallow layer of water.
Explain why the baseline is drawn in pencil and not in ink.
Explain why the baseline must be above the level of the water in the beaker.
The solvent front moves 8.0 cm from the baseline. The table shows the distance moved by each spot.
| Colouring | Distance moved by spot / cm |
|---|---|
| E | 2.4 |
| F | 4.0 |
| G | 5.6 |
| H | 6.4 |
| sweet colouring | 2.4 and 5.6 |
Calculate the RfR_{f}Rf value of colouring E. Use the equation: Rf=distance moved by substancedistance moved by solvent\displaystyle R_{f} = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}Rf=distance moved by solventdistance moved by substance
Deduce which of the known colourings are in the sweet colouring.
Name the stationary phase in this experiment.
48 exam-style questions on Edexcel GCSE Chemistry 3.2 Methods of separating and purifying substances, covering 3.2.1 Pure substances and mixtures, 3.2.2 Techniques for separating mixtures, 3.2.3 Paper chromatography and Rf values, and 3.2.4 Making water potable. Each one has a worked solution and a mark scheme showing where the marks go.