Making a soluble salt — required practical
AQA GCSE Chemistry
Prepare pure, dry crystals of a soluble salt (e.g. copper sulfate) from an insoluble base and an acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Method
- Warm dilute sulfuric acid, then add copper oxide a little at a time until it is in excess (no more dissolves).
- Filter to remove the excess copper oxide, leaving copper sulfate solution.
- Heat the solution to evaporate some water (to the point of crystallisation), then leave to cool and crystallise.
- Pat the crystals dry with filter paper.
Results & analysis
Adding the base in excess ensures all the acid reacts; filtering removes the unreacted excess; slow crystallisation gives pure crystals.
Risk assessment
- Wear eye protection; take care heating acid — do not evaporate to dryness (spitting).
Improving accuracy & reliability
- Add the base until it is in excess so no acid remains.
- Do not evaporate to dryness — leave to crystallise slowly for better crystals.
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