A forensic scientist uses paper chromatography to compare the ink on a ransom note with the inks in three pens, A, B and C. The same solvent is used each time. The chromatography paper is placed in a tank that is covered with a lid.
Explain why a lid is placed on the tank.
The ink from the note gives two spots. One spot has an RfR_{f}Rf value of 0.30. The scientist carries out four runs to find the RfR_{f}Rf value of the other spot. The table shows the results.
| Run | Distance moved by spot / cm | Distance moved by solvent front / cm |
|---|---|---|
| 1 | 3.3 | 6.0 |
| 2 | 4.4 | 8.0 |
| 3 | 3.9 | 6.0 |
| 4 | 5.5 | 10.0 |
Determine the mean RfR_{f}Rf value of this spot. You must show your working.
Suggest one cause of the anomalous result.
The table shows the RfR_{f}Rf values of the dyes in the three pens.
| Pen | RfR_{f}Rf values of dyes |
|---|---|
| A | 0.30 and 0.55 |
| B | 0.55 and 0.80 |
| C | 0.30 and 0.65 |
Deduce which pen was used to write the ransom note. Justify your answer.
Evaluate whether this evidence proves that the pen you identified was used to write the note.
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.