A jeweller claims that a sample of metal is pure gold. A technician heats the sample and records the temperature at which it begins to melt and the temperature at which it is completely melted. The technician finds that the sample begins to melt at 1051 °C and is completely melted at 1068 °C. The melting point of pure gold is 1064 °C. Explain what the technician's results tell us about the sample, and describe one further test the technician could carry out to find out more about what the sample contains.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The sample is not a pure substance – it is a mixture.
Because the sample melts over a temperature range (1051 °C to 1068 °C) rather than at a single, sharp temperature.
A pure substance has a sharp melting point – it melts at one specific temperature (1064 °C for gold).
The technician could carry out paper chromatography on a dissolved sample.
This would separate and identify the different substances or components present in the mixture.
Because the sample melts over a temperature range (1051 °C to 1068 °C) rather than at a single, sharp temperature.
A pure substance has a sharp melting point – it melts at one specific temperature (1064 °C for gold).
The technician could carry out paper chromatography on a dissolved sample.
This would separate and identify the different substances or components present in the mixture.
Examiner tips
- Use the word ‘mixture’ to show the sample is not pure. Explain the melting‑point range versus a sharp point. Name the technique (paper chromatography) and its purpose – separation/identification.
- common_mistakes
- :
- Saying the sample is ‘impure’ without explaining the melting‑point range. Forgetting to name the technique correctly or its purpose. Using a technique that does not separate components, e.g. filtration.
Mark scheme (5 marks)
- The sample is not a pure substance / is a mixture
- Because it melts over a range of temperatures rather than at a single/sharp melting point
- A pure substance has a sharp melting point (melts at one specific temperature)
- Carry out paper chromatography on a dissolved sample (correct method named)
- This would separate / identify the different substances / components present in the mixture (correct purpose of the further test)
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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