Chemistry · Practical thinking
Why can’t a filter remove dissolved salt?
Use a particle model to choose between filtration and crystallisation.
The short answer
Filtration separates an insoluble solid from a liquid. Dissolved substances pass through ordinary filter paper with the solvent, so filtration does not separate dissolved salt from water.
Conceptual illustration. Colours distinguish parts of the model.
Where the confusion starts
Clear does not mean pure. A salt solution can be transparent while containing dissolved ions. A useful choice of method depends on whether the solid is dissolved, and on which material you want to recover.
WORK IT THROUGH
Separate a mixture of sand, salt and water.
- Filter the mixture. Insoluble sand remains as residue, while salt solution passes through as filtrate.
- To obtain salt crystals, evaporate some water gently to concentrate the filtrate, then allow it to cool and crystallise.
- Collect and dry the crystals. To recover the water instead, consider distillation, which includes condensation of the vapour.
CHECK YOUR UNDERSTANDING
Sugar is fully dissolved in water. Will ordinary filter paper collect the sugar?
Make a prediction or write an answer before opening the explanation.
Reveal the explanation
No. The dissolved sugar passes through with the water. Choose a method based on solubility and what you need to recover; filtration alone is insufficient.
SEE IT. CHANGE IT. EXPLAIN IT.
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