CIE iGCSE Co-Ordinated Science C12.4 Separation and purification Exam Style Questions Paper 4
Question
A student investigates the colour in a leaf. He crushes the leaf in a solvent to extract the coloured compounds. Fig. 5.1 shows the apparatus he uses.
(a) (i) Name the process he uses to remove the solids from the cloudy green liquid to obtain a clear green solution.
▶️Answer/Explanation
Filtration
Explanation: Filtration separates insoluble solids (leaf tissue) from the liquid solvent containing dissolved pigments, using filter paper that traps solid particles while allowing the solution to pass through.
(ii) Describe how the process he uses produces a clear green solution.
▶️Answer/Explanation
The filter paper retains solid particles while allowing the dissolved pigments to pass through, resulting in a particle-free solution.
Explanation: The pore size in filter paper is small enough to block plant cell debris but large enough to let pigment molecules through, clarifying the solution while retaining the green color.
(b) The student uses paper chromatography to separate the compounds which give the solution its green colour. He draws a pencil line on a strip of chromatography paper and places a drop of the green solution on the pencil line. He dips the paper into a solvent. Fig. 5.2 is an incomplete diagram of the apparatus he uses.
(i) Draw a line on Fig. 5.2 to show the surface of the solvent in the beaker.
▶️Answer/Explanation
A horizontal line should be drawn:
– Below the pencil line/spot
– Above the bottom of the paper
Explanation: The solvent level must be below the pigment spot to prevent immediate dissolution of samples into the solvent reservoir, allowing proper capillary action.
(ii) Fig. 5.3 shows the chromatogram he obtains.
Table 5.1 lists the Rf values of the coloured compounds on the chromatogram.
| coloured compound | Rf |
|---|---|
| carotene | 0.91 |
| chlorophyll A | 0.42 |
| chlorophyll B | 0.16 |
| xanthophyll | 0.77 |
Use Table 5.1 to identify the yellow compound. Explain how you obtained your answer.
▶️Answer/Explanation
Yellow compound: Xanthophyll
Explanation: It has the second highest Rf value (0.77), matching the position of the second spot from the top on the chromatogram which appears yellow.
Additional note: Carotene (orange) would have higher Rf (0.91), while chlorophylls (green) have lower Rf values.
(iii) Describe how the student can obtain a pure, dry sample of the orange compound from the chromatogram.
▶️Answer/Explanation
1. Cut out the orange band with scissors
2. Place in a small volume of solvent to dissolve the pigment
3. Filter and evaporate the solvent to obtain dry carotene
Explanation: This process isolates the desired compound by:
– Physically separating the pigment band
– Re-dissolving in fresh solvent
– Removing solvent through evaporation (e.g., using a water bath)