Making a pure, dry soluble salt
GCSE Chemistry (8462) · Required practical 1 — method, variables, the marks examiners report students losing.
Prepare pure, dry crystals of a soluble salt (e.g. copper sulfate) from an insoluble base and a dilute acid.
Apparatus
- Dilute sulfuric acid
- Copper(II) oxide (an insoluble base)
- Beaker, glass rod, Bunsen burner, tripod and gauze
- Filter funnel and filter paper
- Evaporating basin and water bath
Method
- 1Warm dilute sulfuric acid gently in a beaker with the Bunsen — do not boil.
- 2Add copper(II) oxide a little at a time, stirring, until some remains unreacted (it is in excess) — this makes sure all the acid has reacted.
- 3Filter the mixture to remove the excess copper(II) oxide, leaving blue copper sulfate solution.
- 4Pour the solution into an evaporating basin and heat gently over a water bath to evaporate about half the water (to the point of crystallisation).
- 5Leave the solution to cool slowly so crystals form.
- 6Remove the crystals and pat them dry between filter paper.
Results & processing
- You obtain blue crystals of copper sulfate.
- Slower cooling gives larger, purer crystals.
Where students lose marks
Boiling the solution dry to remove all the water.
Fix: Evaporate only to the point of crystallisation, then let it cool — boiling dry gives a powder and can decompose the salt / drive off water of crystallisation.
Not adding the base in excess.
Fix: Add copper(II) oxide until some is left unreacted, so all the acid is used up; the excess solid is then removed by filtering.
Evaporating before filtering out the excess base.
Fix: Filter first — otherwise the unreacted insoluble base contaminates the crystals.
Improve the method
- Use a water bath rather than a direct flame for gentle, even evaporation.
- Cool slowly for larger, purer crystals; pat dry rather than heating the crystals.
Try it — exam-style
Explain why the copper(II) oxide is added until it is in excess.
A student heats the copper sulfate solution strongly until all the liquid has gone. Explain why this reduces the quality of the crystals.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on making a pure, dry soluble salt?
The six-mark 'describe how to prepare a salt' question is pure method recall — I drill the exact sequence examiners want, and your first lesson is free.