Paper chromatography
4 marks · NormalA student uses paper chromatography to separate a mixture of coloured dyes. The chromatogram below shows the results. The student also spotted two known dyes, X and Y, on the same piece of chromatography paper.
| Sample | Distance moved from origin (cm) |
|---|---|
| Mixture (green) | 4.8 |
| Dye X (blue) | 1.7 |
| Dye Y (orange) | 4.8 |
| Solvent front | 6.0 |
State which dye(s), X or Y, is present in the mixture. Give a reason for your answer.
Explain why the blue spot (Dye X) did not reach as far up the paper as the orange spot (Dye Y).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Dye Y is present in the mixture because its spot moved the same distance from the origin as the green spot in the mixture (4.8 cm). Dye X is not present, as its spot (1.7 cm) does not match any spot in the mixture.
(b) The blue spot (Dye X) did not reach as far up the paper as the orange spot (Dye Y) because it is either less soluble in the solvent or has a greater affinity for the paper (sticks more strongly to the paper), so it does not travel as far.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies that Dye Y is present in the mixture (or both if correctly justified)
B2 (a) · 1 mark · States that Dye Y is present because it moved the same distance as the spot in the mixture OR R_f values match
B3 (b) · 1 mark · States blue spot did not travel as far because it is less soluble OR has more attraction (affinity) to the paper
B4 (b) · 1 mark · States either solubility (less soluble) or affinity for the paper (sticks more to the paper) with correct explanation
Worked solution
a) Dye Y is present in the mixture because the green spot in the mixture and the orange spot (Dye Y) both moved 4.8 cm from the origin. Dye X is not present as its spot moved 1.7 cm, which does not match any spot from the mixture.
b) The blue spot (Dye X) did not travel as far up the paper as the orange spot (Dye Y) because it is either less soluble in the solvent or has a higher affinity for the paper, causing it to stick more and move less.