A technician uses paper chromatography to test an indicator mixture that is known to contain two coloured substances, P and Q.
Spots of the indicator mixture, pure P and pure Q are placed on a pencil start line. The bottom of the paper is dipped into liquid W, with the level of W below the start line.
The indicator mixture produces only one spot. The table shows the distances measured from the start line.
| Material | Distance moved from the start line in cm |
|---|---|
| Liquid W | 8.0 |
| Spot from the indicator mixture | 5.6 |
| Spot from pure P | 5.6 |
| Spot from pure Q | 5.6 |
Explain how liquid W causes substances P and Q to move up the paper.
Calculate the Rf value of substance P.
Explain why liquid W has not separated P from Q in the indicator mixture. Refer to the mobile phase and the stationary phase in your answer.
Suggest one change to the method that could separate P and Q.
102 exam-style questions on OCR GCSE Chemistry 2.1 Purity and separating mixtures, covering 2.1.1 Pure substances and melting point data, 2.1.2 Relative formula mass and empirical formula, 2.1.3 Formulations as useful mixtures, 2.1.4 Techniques for separating mixtures, and 2.1.5 Chromatography and Rf values. Each one has a worked solution and a mark scheme showing where the marks go.