At a community food laboratory, a student checks whether a black icing colouring contains more than one dye. A drop of colouring solution is dried at the st...
Question 1 Report
At a community food laboratory, a student checks whether a black icing colouring contains more than one dye. A drop of colouring solution is dried at the start line on a chromatography strip. The strip stands in water, with the liquid level below the start line. After the water has travelled, the strip is removed and marked immediately. Fig. 1 is the student's chromatogram. Table 1 gives reference Rf values obtained using the same water and paper.
Fig. 1
| reference dye | Rf |
|---|---|
| yellow | 0.25 |
| green | 0.50 |
| blue | 0.75 |
Table 1
(a) Name the method used to separate the dyes. [1]
(b) State why a pencil, rather than an ink pen, is used for the start line. [1]
(c) Calculate the Rf value of the yellow spot. Show your working. [3]
(d) Use Table 1 to identify the two dyes in the black colouring. [2]
(e) State what Fig. 1 shows about the black colouring. [2]
(f) Explain why the water level must remain below the start line at the beginning of the experiment. [3]
(g) Suggest three changes that would make the conclusion about the dyes more reliable. [3]
Answer Details
(a) The method is paper chromatography. [1]
(b) Pencil is used because graphite does not dissolve in the water solvent or travel up the paper. Ink would dissolve and could produce extra spots. [1]
(c) \[R_f=\frac{\text{distance travelled by dye}}{\text{distance travelled by solvent front}}\]
\[R_f=\frac{2.0\text{ cm}}{8.0\text{ cm}}=0.25\]
The yellow spot has \(R_f=\mathbf{0.25}\). [3]
(d) The dyes are yellow, with \(R_f=0.25\), and blue, with \(R_f=0.75\). [2]
(e) The black colouring is a mixture because it separates into two spots, showing that it contains two dyes. [2]
(f) If the water level covers the start line, the dye dissolves directly into the water. It then washes off the paper instead of separating as the solvent rises up the paper. [3]
(g) Three improvements are to repeat the chromatography, run reference dyes alongside the sample, and use a sealed container to reduce solvent evaporation. Keeping solvent volume the same, using fresh identical paper, and measuring distances more accurately are also valid reliability improvements. [3]
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