Elements, Compounds and Mixtures
6 marks · NormalA student finds a white powder in the laboratory labelled 'Sample X'. She wants to check if it is a pure substance or a mixture, and then investigate its components using paper chromatography.
Describe how the student could use melting point measurements to determine whether Sample X is pure or a mixture.
The student performs a paper chromatography experiment with a spot of Sample X and three reference dyes A, B and C on the same chromatography paper. The chromatogram is developed using a suitable solvent.
The diagram below shows the results.
The solvent front has moved 6.0 cm from the pencil line. The spots are found at the following distances above the pencil line: Sample X: 2.0 cm, 4.5 cm Dye A: 2.0 cm Dye B: 4.5 cm Dye C: 3.7 cm
How many different dyes are present in Sample X? Explain your answer.
State which reference dyes are found in Sample X. Give your reasoning.
Calculate the R_f value for the spot at 3.7 cm. Show your working.
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) A pure substance has a sharp melting point, while a mixture melts over a range of temperatures. The student would heat the substance and measure its melting point. If it melts at a specific, constant temperature, it is pure; if it melts over a range, it is a mixture.
(b.i) There are two dyes in sample X, as there are two spots at different distances from the pencil line.
(b.ii) Sample X contains dyes A and B. The spots at 2.0 cm and 4.5 cm match the positions of dyes A and B on the chromatogram.
(c) Rf = 3.7 / 6.0 = 0.62 · must show Rf calculation
Common mistakes: Stating that a mixture has a sharp melting point; Mismatching dye spots by distance rather than colour and distance; Calculating Rf by dividing the wrong way round
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that a pure substance has a sharp (fixed) melting point, while a mixture melts over a range.
A1 (a) · 1 mark · Describes measuring the melting point/range to determine purity.
B1 (b(i)) · 1 mark · Correctly identifies number of dyes in Sample X from number of spots.
B2 (b(ii)) · 1 mark · Correctly matches Sample X spots to reference dyes with clear reasoning.
M2 (c) · 1 mark · States or uses correct Rf formula: distance moved by spot/distance moved by solvent front, with matching numbers.
A2 (c) · 1 mark · Correctly calculates Rf value (0.62, accept 0.62–0.62 with rounding). (follow-through allowed) (follow-through)
Worked solution
a) The student heats Sample X slowly and measures its melting point. If it melts at one constant temperature, it is a pure substance. If it melts over a temperature range, it is a mixture.
b) i) There are two different dyes present in Sample X, as two distinct spots are seen on the chromatogram.
ii) Dyes A and B are found in Sample X. Their spots are at the same distances (2.0 cm for A, 4.5 cm for B) as spots from Sample X.
c) The R_f value for the spot at 3.7 cm is calculated as:
R_f = 0.62