AQA GCSE Chemistry · Chemical Changes
Making salts required practical
Prepare a pure, dry sample of a soluble salt from an insoluble metal oxide and a dilute acid.
Copper oxide reacts with warm sulfuric acid to make blue copper sulfate solution. You add the oxide in excess, filter off what’s left, then crystallise the salt.
Equipment
- Dilute sulfuric acid and copper(II) oxide powder
- Beaker, spatula and glass rod
- Bunsen burner, tripod and gauze (or a water bath)
- Filter funnel and filter paper
- Evaporating basin and conical flask
- Measuring cylinder
Variables
- Independent you change
- Not a fair-test practical: you follow a method to make a product
- Dependent you measure
- A pure, dry sample of copper sulfate crystals
- Control you keep the same
- Add the oxide in excess so all the acid reacts
- Warm the acid gently, don’t boil it
- Heat the solution only until crystals start to form
Method
- 1
Measure 40 cm³ of dilute sulfuric acid into a beaker and warm it gently (to about 50 °C).
- 2
Add copper oxide a spatula at a time, stirring, until no more dissolves and some black solid is left. The oxide is now in excess.
- 3
Filter the mixture to remove the excess copper oxide. Collect the blue filtrate.
- 4
Pour the filtrate into an evaporating basin and heat it over a water bath until about half the water has evaporated.
- 5
Leave the solution in a warm place to cool slowly, so crystals form.
- 6
Remove the crystals and pat them dry between filter papers.
Worked example
What you see at each stage.
| Stage | Observation | Why |
|---|---|---|
| Adding copper oxide | Black powder disappears; solution turns blue | Copper sulfate is forming |
| Oxide in excess | Black solid stays at the bottom | All the acid has reacted |
| After filtering | Clear blue filtrate | Excess oxide stays in the filter paper |
| Cooling | Blue crystals form | Less salt can stay dissolved as the water cools |
Working
copper oxide + sulfuric acid → copper sulfate + water
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
Conclusion
Adding the insoluble base in excess makes sure all the acid is used up, so the only thing in solution is copper sulfate. Filtering removes the leftover oxide, and crystallising gives a pure, dry salt.
Risks
- Sulfuric acid is an irritantWear eye protection; wash splashes off skin.
- Copper oxide and copper sulfate are harmfulDon’t eat in the lab; wash hands afterwards.
- Hot solution can spit when heatedHeat over a water bath, gently, and stop before it dries out.
Common mistakes
- Not adding enough copper oxide. Leftover acid would end up in your crystals; keep adding until some solid won’t dissolve.
- Heating the solution to dryness. This gives a powder instead of crystals and can break down the salt.
- Mixing up filtrate and residue. The salt is in the filtrate (the liquid that passes through); the excess oxide is the residue.
Better results
- Cool the solution slowly to get larger crystals.
- Warm the acid first so the reaction is faster and more oxide reacts.
- Rinse the crystals with a little cold distilled water before drying them.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Chemical Changes flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.