A scientist is testing a new sunscreen product. The scientist wants to find out whether the sunscreen is a pure substance or a formulation, and then uses crystallisation to obtain a solid component from it. Describe how the scientist could determine whether the sunscreen is a pure substance or a mixture, and outline the steps involved in crystallisation to obtain a solid from the sunscreen solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A scientist is investigating the composition of a sunscreen product. The sunscreen is dissolved in water to produce a solution. The scientist suspects the sunscreen is a formulation rather than a pure substance.
Model answer (5 marks)
1. Measure the melting point of a sample of the sunscreen.
2. Compare the measured melting point to literature values for known pure substances.
3. If the sunscreen has a sharp, specific melting point, it is likely a pure substance; if it melts over a range of temperatures, it is a mixture.
4. To isolate a solid component, heat the sunscreen solution in an open container until the solvent evaporates and a saturated solution remains.
5. Cool the saturated solution slowly so crystals grow, then filter the crystals from the remaining liquid and dry them to obtain the solid component.
2. Compare the measured melting point to literature values for known pure substances.
3. If the sunscreen has a sharp, specific melting point, it is likely a pure substance; if it melts over a range of temperatures, it is a mixture.
4. To isolate a solid component, heat the sunscreen solution in an open container until the solvent evaporates and a saturated solution remains.
5. Cool the saturated solution slowly so crystals grow, then filter the crystals from the remaining liquid and dry them to obtain the solid component.
Examiner tips
- Use the command word ‘describe’ – give a clear, step‑by‑step explanation. Include both the melting‑point test and the crystallisation procedure. Mention comparison with literature data to justify purity. Show the cooling step before filtration to earn the full 5 marks.
Common mistakes
- Failing to mention the comparison with literature melting points. Skipping the cooling step before filtration. Using vague terms like ‘separate the solid’ without describing crystallisation steps.
Mark scheme (5 marks)
- A pure substance has a sharp/specific melting point whereas a mixture melts over a range of temperatures
- Measure/determine the melting point of the sunscreen and compare it to known/data book values
- A formulation is a mixture designed for a particular use, made up of exact quantities of pure substances — the sunscreen is likely a formulation because it contains multiple components for a specific purpose
- Heat the sunscreen solution in an open container to evaporate the solvent until crystals begin to form / a saturated solution is produced
- Allow the solution to cool so that crystals grow, then filter the crystals from the solution and allow them to dry
Key terms in this question
pure substance · formulation · crystallisation · mixture
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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