A student has a mixture of sand and salt dissolved in water. Describe how the student could obtain a pure, dry sample of salt from this mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Filter the mixture through a filter paper to remove the sand, collecting the filtrate which contains the dissolved salt. Evaporate the filtrate in a shallow dish or on a hot plate until all the water has left, leaving a solid residue of salt. Dry the remaining solid in a warm oven or by air‑drying to obtain a pure, dry sample of salt.
Examiner tips
- Use the word ‘filter’ to show you know the first step; mention ‘filter paper’ and ‘insoluble residue’.
- Show the sequence: filter → collect filtrate → evaporate → dry the residue.
- Include that the salt is left as a solid after water removal.
Common mistakes
- Not specifying that the sand is removed by filtration.
- Failing to mention that the filtrate must be evaporated to recover the salt.
- Leaving the salt wet or not indicating it must be dried before grading.
Mark scheme (4 marks)
- Filter the mixture to remove the sand (insoluble residue)
- Collect the filtrate (salt solution) that passes through the filter
- Evaporate the filtrate / salt solution to remove the water
- Allow the remaining solid to dry / leave in a warm oven to obtain dry salt crystals
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →