7.1.4Practical Skills
Production of Salts — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 6
Part 1 of 5The chemistry
Acid plus insoluble base
- Acid plus insoluble base An acid reacts with a metal oxide or carbonate to form a salt and water (and carbon dioxide for a carbonate).
- Copper(II) oxide + sulfuric acid → copper(II) sulfate + water: CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l).
Why use an excess of the base
- Why use an excess of the base Adding more solid than will react guarantees that all the acid has been used up.
- The leftover solid is easy to see and can be removed by filtering, which leaves a neutral solution.
Part 2 of 5Method for pure dry copper(II) sulfate crystals
1
Measure about 50 cm3 of dilute sulfuric acid into a beaker and warm it gently with a Bunsen burner, but do not allow it to boil.
2
Add small amounts of copper(II) oxide powder using a spatula, stirring with a glass rod, until no more dissolves and a black suspension remains.
3
Filter the mixture through a filter funnel and paper to remove the excess copper(II) oxide, collecting the blue filtrate in an evaporating basin.
4
Heat the filtrate gently, for example over a water bath, to evaporate some of the water until the solution is saturated.
5
Test for saturation by dipping a cold glass rod into the solution; crystals forming on the rod show that it is ready.
6
Leave the solution in a warm place to cool and crystallise, then pour away the remaining solution and dry the crystals between filter paper.
Part 3 of 5Apparatus and what each item does
| Apparatus | Purpose |
|---|---|
| Measuring cylinder | Measures the volume of acid accurately |
| Beaker and Bunsen burner with tripod, gauze and heatproof mat | Warms the acid to speed up the reaction |
| Spatula and glass rod | Adds the solid and stirs the mixture |
| Filter funnel, filter paper and conical flask | Removes the unreacted solid |
| Evaporating basin | Allows water to evaporate so the solution becomes saturated |
Part 4 of 5Safety
Chemical hazards
- Chemical hazards Dilute sulfuric acid is an irritant;
- copper(II) oxide and copper(II) sulfate are harmful and toxic to aquatic life.
- Avoid breathing in the oxide powder and avoid skin contact.
Precautions
- Precautions Wear safety goggles throughout and tie back long hair.
- Do not let the acid boil, and use a water bath rather than a naked flame for evaporation to prevent spitting.
Part 5 of 5Results and conclusion
Expected result
- Expected result Bright blue crystals of hydrated copper(II) sulfate with a regular shape.
- The regular crystal shape reflects the ordered arrangement of ions in the ionic lattice.
Improving the yield
- Improving the yield Do not evaporate until completely dry, because heating strongly removes the water of crystallisation and leaves a white powder.
- Crystallise slowly for larger, better-formed crystals.
Key terms
| Term | Meaning |
|---|---|
| Salt | An ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. |
| Excess | More of a reactant than is needed to react with the other reactant. |
| Filtrate | The liquid that passes through the filter paper. |
| Saturated solution | A solution that cannot dissolve any more solute at that temperature. |
| Crystallisation | The formation of solid crystals from a saturated solution as it cools or evaporates. |
Avoid these mistakes
1
Adding too little baseif any acid remains, the salt is contaminated and acid fumes may form during evaporation.
2
Boiling the solution dry
- this destroys the crystals;
- evaporate only until the solution is saturated.
3
Forgetting to filter off the excess solid before evaporating.
4
Heating the acid until it boilswarm it gently only.
5
Describing the product as copper sulfate crystals without noting they are blue because they contain water.
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