Elements, Compounds and Mixtures
6 marks · NormalA research chemist is given three samples labelled A, B, and C. She wants to classify them and analyse their composition using paper chromatography.
Sample A consists only of nitrogen gas. Sample B contains sodium chloride crystals. Sample C is a solution made by dissolving food dye in water. Classify each sample as an element, compound, or mixture.
The chemist uses paper chromatography to separate the dyes in Sample C. After running the experiment, she observes three coloured spots at different distances from the pencil line. What does this result tell you about the composition of Sample C?
Explain how you could use the chromatogram to identify if Sample C contains the same dyes as a reference mixture of known dyes.
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.
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(a) A: element; B: compound; C: mixture · must show A = element, B = compound, C = mixture
(b) Sample C is a mixture (of at least three dyes) · must show mixture, at least three dyes present
(c) Compare the positions and colours of the spots from Sample C with those of the known dyes on the same chromatogram. If a spot from Sample C matches both the colour and distance (Rf) of a dye in the reference mixture, that dye is present in Sample C. · must show compare spots on same paper, colour and distance/Rf must match
Common mistakes: Mixing up 'element' and 'compound' definitions; not mentioning spot distance as a criterion for identity.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly classifies A as an element.
B2 (a) · 1 mark · Correctly classifies B as a compound.
B3 (a) · 1 mark · Correctly classifies C as a mixture.
B4 (b) · 1 mark · Recognises that Sample C is a mixture (of at least three dyes).
M1 (c) · 1 mark · Refers to comparing position (distance/Rf) and/or colours of spots between C and reference dyes.
A1 (c) · 1 mark · States that spots matching both position and colour indicate the dye’s presence in Sample C.
Worked solution
a) Sample A is nitrogen gas, which is an element. Sample B is sodium chloride (NaCl), a chemical compound (made from sodium and chlorine atoms in fixed proportions). Sample C is a solution of dye in water, which is a mixture (since the dyes and water are not chemically combined).
b) The observation of three separate coloured spots means that Sample C is made from a mixture of at least three different dyes; each dye separates based on its solubility and affinity for the paper.
c) To identify if Sample C contains the same dyes as a reference mixture, spot the references and Sample C on the same baseline and run chromatography together. If a spot in C matches both the colour and position (or R_f value) of a reference dye, it means that dye is present in Sample C.