A forensic scientist investigates a forged letter. The scientist compares the ink on the letter with the inks from three pens, P, Q and R.
The scientist uses paper chromatography with water as the solvent.
| Ink | Rf values of spots in water |
|---|---|
| letter | 0.21, 0.58 |
| P | 0.21, 0.58 |
| Q | 0.21, 0.58 |
| R | 0.35, 0.58, 0.80 |
The scientist then repeats the chromatography using ethanol as the solvent.
| Ink | Rf values of spots in ethanol |
|---|---|
| letter | 0.15, ?, 0.71 |
| P | 0.15, 0.71 |
| Q | 0.15, 0.47, 0.71 |
| R | 0.33, 0.47, 0.90 |
Explain why ink R could not have been used to write the letter.
On the ethanol chromatogram for the letter, the solvent moved 7.5 cm. The middle spot moved 3.5 cm.
Calculate the Rf value of the middle spot. Give your answer to 2 decimal places.
The letter ink gives two spots in water but three spots in ethanol.
Explain why.
The scientist concludes, "Pen Q was used to write the letter."
Evaluate this conclusion. Use the data from both solvents.
Explain why scientists compare Rf values rather than the distances moved by the spots.
51 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.