A technician is testing a sample of a yellow paint pigment to check whether it is a pure substance or a mixture. She uses paper chromatography with two different solvents. In both solvents, the sample produces only a single spot on the paper. Explain what these results tell the technician about the yellow pigment, and describe one further test she could carry out to confirm that the pigment is a pure substance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The single spot in both solvents shows that the pigment does not separate into components, so it is a pure substance rather than a mixture. A mixture would give several spots.
A further test is to determine the melting point. A pure pigment will have a sharp, specific melting point, whereas a mixture will melt over a range of temperatures.
A further test is to determine the melting point. A pure pigment will have a sharp, specific melting point, whereas a mixture will melt over a range of temperatures.
Examiner tips
- Use the word ‘shows’ or ‘indicates’ to link the observation to the conclusion.
- Mention that a mixture would give multiple spots.
- State that a pure substance has a sharp melting point.
- Explain that a mixture melts over a range of temperatures.
Common mistakes
- Saying the pigment is a mixture because of a single spot.
- Not linking the single spot to purity.
- Failing to explain the melting point test.
Mark scheme (4 marks)
- A pure compound produces a single spot in all solvents (so the single spot in both solvents suggests the pigment is a pure substance / not a mixture)
- A mixture would separate into more than one spot, so the absence of multiple spots suggests the pigment is not a mixture
- Further test: measure / determine the melting point of the pigment
- A pure substance has a sharp / specific melting point, whereas a mixture melts over a range of temperatures
Key terms in this question
pure substance · mixture · paper chromatography
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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