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This lesson covers the Core Practical for preparing a pure, dry sample of a soluble salt as required by the Edexcel GCSE Combined Science specification (1SC0). This is a required practical that you must know in detail, including the method, the reasoning behind each step, and how to evaluate results.
To prepare a pure, dry sample of a soluble salt (e.g. copper sulfate) by reacting an acid with an excess of an insoluble base or insoluble carbonate.
This practical uses the reaction between sulfuric acid and copper oxide to make copper sulfate crystals.
copper oxide + sulfuric acid → copper sulfate + water
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
Copper oxide is an insoluble base — it does not dissolve in water but does react with acid.
| Item | Purpose |
|---|---|
| Beaker (250 cm³) | To carry out the reaction |
| Measuring cylinder | To measure the acid |
| Glass stirring rod | To stir the mixture |
| Bunsen burner, tripod, gauze (or water bath) | To warm the acid |
| Filter funnel and filter paper | To remove excess copper oxide |
| Conical flask or beaker | To collect the filtrate |
| Evaporating basin | To evaporate the filtrate |
| Spatula | To add copper oxide |
| Safety goggles | Eye protection |
graph TD
A["1. Measure 25 cm³ of dilute<br/>sulfuric acid into a beaker"] --> B["2. Warm the acid gently<br/>using a Bunsen burner<br/>or water bath"]
B --> C["3. Add a spatula of<br/>copper oxide powder<br/>and stir"]
C --> D["4. Continue adding copper oxide<br/>until it is in EXCESS<br/>(black powder remains undissolved)"]
D --> E["5. Filter the mixture<br/>to remove excess<br/>copper oxide"]
E --> F["6. Transfer the blue filtrate<br/>to an evaporating basin"]
F --> G["7. Heat gently on a<br/>water bath to evaporate<br/>some water"]
G --> H["8. Stop heating when<br/>crystals start to form<br/>at the edges"]
H --> I["9. Leave to cool and<br/>crystallise slowly"]
I --> J["10. Pat crystals dry<br/>with filter paper"]
style A fill:#2980b9,color:#fff
style B fill:#2980b9,color:#fff
style C fill:#27ae60,color:#fff
style D fill:#27ae60,color:#fff
style E fill:#e67e22,color:#fff
style F fill:#e67e22,color:#fff
style G fill:#8e44ad,color:#fff
style H fill:#8e44ad,color:#fff
style I fill:#c0392b,color:#fff
style J fill:#c0392b,color:#fff
Measure the acid: Use a measuring cylinder to add 25 cm³ of dilute sulfuric acid (approximately 1 mol/dm³) to a beaker.
Warm the acid: Place the beaker on a tripod and gauze, and warm gently using a Bunsen burner. Alternatively, use a water bath. Warming increases the rate of reaction. Do not boil.
Add copper oxide: Using a spatula, add a small amount of copper oxide powder to the warm acid. Stir with a glass rod. The black powder will dissolve, and the solution will turn blue (copper sulfate forming).
Add excess: Keep adding copper oxide, a little at a time, stirring after each addition. When the copper oxide is in excess, you will see black powder remaining undissolved at the bottom, even after stirring. This means all the acid has been used up.
Filter: Set up a filter funnel with filter paper over a conical flask. Pour the mixture through. The excess copper oxide (black residue) is trapped by the filter paper. The blue copper sulfate solution (filtrate) passes through into the flask.
Transfer to an evaporating basin: Pour the blue filtrate into a clean evaporating basin.
Evaporate: Place the evaporating basin on a water bath (a beaker of boiling water) and heat gently. This slowly evaporates the water. Do not use a direct flame — this can cause spitting and may decompose the salt.
Stop heating: When you see crystals starting to form at the edges of the solution, or when a glass rod dipped in the solution and held up forms crystals on its surface, stop heating.
Crystallise: Leave the saturated solution to cool slowly at room temperature. Blue crystals of copper sulfate will form.
Dry: Gently remove the crystals and pat them dry between two pieces of filter paper. Alternatively, leave in a warm oven at low temperature.
Exam Tip: The most commonly missed step is explaining why you add an excess of the insoluble base. The answer is: to ensure all the acid has reacted, so the salt product is not contaminated with unreacted acid.
Warming increases the rate of reaction, so the copper oxide dissolves more quickly.
To ensure that all the acid has reacted. If acid remained, the final product would be impure (contaminated with acid).
When black solid remains undissolved in the solution after stirring — this means there is no more acid left to react with.
To remove the excess insoluble solid (copper oxide). The filtrate contains only the dissolved salt (copper sulfate) and water.
If you evaporate all the water, the crystals will be small, powdery and impure. Leaving the solution to cool slowly produces larger, more regular crystals.
Exam Tip: When describing this practical, always mention: warming the acid, adding excess, filtering, evaporating on a water bath, and leaving to crystallise slowly. These are the key steps examiners look for.
| Step | Observation |
|---|---|
| Adding CuO to H₂SO₄ | Black powder dissolves; solution turns blue |
| Excess CuO | Black powder remains undissolved at the bottom |
| Filtering | Blue filtrate passes through; black residue stays on filter paper |
| Evaporation | Blue solution becomes more concentrated |
| Crystallisation | Blue crystals of copper sulfate form |
| Hazard | Risk | Control Measure |
|---|---|---|
| Sulfuric acid | Irritant/corrosive | Wear safety goggles; wash skin immediately if contact occurs |
| Copper oxide | Harmful if inhaled | Avoid creating dust; add carefully |
| Hot liquids | Burns | Use a water bath; handle with care; do not point evaporating basin towards anyone |
| Bunsen burner | Burns | Tie back hair; place on heat-proof mat |
When evaluating your crystals, consider:
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