Required practical 8 · Chemistry · Chemistry Paper 1, both tiers
Making a soluble salt
Prepare a pure, dry sample of a soluble salt, such as copper sulfate, from an insoluble oxide or carbonate and a dilute acid.
The exam trap. Never heat to dryness, and filter out the excess base before crystallising.
Variables
- Independent (you change)
- (A preparation, not an investigation) the choice of insoluble base and acid
- Dependent (you measure)
- The mass and purity of dry crystals obtained
- Control (you keep the same)
- Adding the base until it is in excess
- Not heating the solution to dryness
- Drying the crystals the same way
Apparatus
- Dilute sulfuric acid and copper oxide
- Beaker, spatula and glass rod
- Water bath or Bunsen burner, tripod and gauze
- Filter funnel and filter paper
- Evaporating basin
- Crystallising dish
Method, and why each step matters
Warm the dilute acid gently in a beaker.
Why Warm acid reacts faster with the oxide.
Add copper oxide a spatula at a time, stirring, until no more dissolves and some solid is left over.
Why Excess base makes sure all the acid is used up.
Filter the mixture to remove the excess copper oxide.
Why The unreacted solid must be removed to leave only salt solution.
Heat the filtrate in an evaporating basin to evaporate about half the water.
Why A more concentrated solution will crystallise as it cools.
Leave the solution to cool and crystallise; do not heat to dryness.
Why Heating to dryness can decompose the salt and gives tiny, impure crystals.
Pat the crystals dry with filter paper or leave them in a warm place.
Why This removes the remaining water.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Dilute sulfuric acid is an irritant. | Wear eye protection. |
| Copper oxide and copper sulfate are harmful. | Avoid skin contact; wash hands. |
| Hot solution can spit when evaporating. | Heat gently and wear eye protection. |
A worked set of results
| Observation | What it shows |
|---|---|
| Black copper oxide dissolves, solution turns blue | Copper sulfate is forming |
| Black solid left over | The copper oxide is in excess |
| Blue crystals form on cooling | The salt crystallises from a saturated solution |
- copper oxide + sulfuric acid → copper sulfate + water.
- With a carbonate, fizzing (carbon dioxide) stops when the acid is used up.
Mistakes that cost marks
Heating the solution to dryness.
Instead Evaporate only part of the water, then leave to crystallise.
Not removing the excess insoluble base before crystallising.
Instead Filter first; the crystals would otherwise contain the oxide.
Using too little base, leaving acid in the product.
Instead Add base until some stays undissolved.
Practice questions on this practical
- A student wants to make pure, dry crystals of copper sulfate from copper oxide (an insoluble black powder) and dilute sulfuric acid.In Acids, salts and pH
- A student makes zinc sulfate crystals by adding excess zinc oxide to dilute sulfuric acid, filtering, and then boiling the filtrate until no liquid remains.In Acids, salts and pH