Elements, Compounds and Mixtures
7 marks · HardA chemist is given two samples labelled X and Y. She suspects that X is a compound and Y is a mixture. She performs paper chromatography on both samples along with three dyes labelled D1, D2, and D3 on the same sheet.
| Substance | Distance moved by spot (cm) |
|---|---|
| Solvent front | 5.0 |
| X | 2.0 |
| Y (spot 1) | 1.0 |
| Y (spot 2) | 3.0 |
| D1 | 1.0 |
| D2 | 2.0 |
| D3 | 3.0 |
Classify samples X and Y as elements, compounds, or mixtures, using the results of the chromatogram. Explain your reasoning.
For each spot found for Y, identify which dye(s), if any, are present in Y.
Calculate the R_f value for the spot found for X.
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) — · must show X classified as a compound (one spot matching a dye), Y classified as a mixture (two spots at different positions)
(b) — · must show Y spot 1 is D1 (both at 1.0 cm), Y spot 2 is D3 (both at 3.0 cm)
(c) 0.40 · must show Rf = 2.0 / 5.0 = 0.40
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly states that X is a compound (based on one spot); Y is a mixture (based on two spots).
M2 (a) · 1 mark · Links the classification of a compound to one substance/spot and mixture to multiple substances/spots.
A1 (a) · 1 mark · Explanation uses evidence from the chromatogram table.
B1 (b) · 1 mark · Identifies Y's spot at 1.0 cm as D1.
B2 (b) · 1 mark · Identifies Y's spot at 3.0 cm as D3.
M3 (c) · 1 mark · Writes or uses correct Rf formula; measures from the origin.
A2 (c) · 1 mark · Calculates Rf for X as 0.40 (accept answers between 0.38–0.42).
Worked solution
a) According to the chromatogram table, sample X gives one spot, while Y gives two spots.
A pure compound usually gives a single spot because it only contains one substance, whereas a mixture gives multiple spots at different positions if more than one component is present.
Therefore, X is a compound and Y is a mixture.
b) Y has a spot at 1.0 cm and a spot at 3.0 cm. Dye D1 is at 1.0 cm and D3 at 3.0 cm.
So, Y contains D1 and D3.
c) R_f value = (distance moved by spot) / (distance moved by solvent front)