Explain why the copper(II) oxide is added until it is in excess.
[2 marks]
Total for this question: 2
Chemistry required practicals
Making a pure, dry soluble salt
AQA 4.4 · RP1
GCSE Chemistry (8462) · Required practical 1 — method, variables, the marks examiners report students losing.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
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
Easy
ORIGINAL
Medium
ORIGINAL
A student heats the copper sulfate solution strongly until all the liquid has gone. Explain why this reduces the quality of the crystals.
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers — never copied from them.
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