A water treatment plant produces drinking water from river water. The river water contains sand particles and dissolved minerals. Describe how filtration and evaporation could be used in sequence to obtain a dry sample of the dissolved minerals from the river water.

OCR GCSE Chemistry A: Gateway Science (J248) — C2.1 Purity and separating mixtures · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

River water is a mixture containing insoluble particles such as sand and soluble dissolved minerals. Water treatment scientists use separation techniques to obtain pure samples of these components.

Model answer (4 marks)

1. Filter the river water through filter paper – the sand is retained by the paper and the clear filtrate passes through.
2. Transfer the filtrate to an evaporating dish.
3. Heat the dish – the water evaporates.
4. The dry mineral crystals remain in the dish.

Examiner tips

State the steps in the correct order: filtration first, then evaporation.
Mention that the sand is retained by the filter paper and the filtrate is heated.
Show that evaporation removes the solvent leaving a dry solid.
Use the exact terms ‘filtration’, ‘filtrate’, ‘evaporation’, ‘solvent’, ‘solid’.

Common mistakes

Writing the steps in the wrong order (evaporation before filtration).
Failing to mention that the sand is retained by the filter paper.
Using vague terms like ‘separate’ instead of ‘filtration’ and ‘evaporation’.

Mark scheme (4 marks)

Filtration is used first to separate the insoluble sand from the river water / the sand is retained by the filter paper and the solution passes through as filtrate
The filtrate / solution containing dissolved minerals is then heated / the solution is placed in an evaporating dish and heated
The solvent (water) evaporates / is driven off, leaving behind the dry mineral crystals / solid
The correct order must be stated: filtration must be performed before evaporation / filtration removes insoluble sand first so that only the solution of dissolved minerals is then evaporated

Key terms in this question

filtration · evaporation