4.2.4Chemical Changes
Soluble Salts — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 9 of 18
Part 1 of 4Choosing the reactants
Acid plus insoluble base
- Acid plus insoluble base A soluble salt can be made by reacting a dilute acid with an insoluble metal oxide, hydroxide or carbonate.
- Copper(II) oxide with sulfuric acid gives copper(II) sulfate.
Using a metal
- Using a metal A metal can also be used, but it must be above hydrogen in the reactivity series and not so reactive that the reaction is dangerous.
- Zinc or magnesium suit this; sodium or potassium do not.
Soluble bases are unsuitable
- Soluble bases are unsuitable With a soluble alkali you cannot tell when to stop and cannot filter off the excess,
- so a different method (titration) is needed.
Part 2 of 4Preparation stages
1
Warm a measured volume of dilute acid in a beaker.
2
Add the insoluble solid a little at a time, stirring, until no more dissolves.
3
Filter the mixture to remove the excess solid; the filtrate is the salt solution.
4
Heat the filtrate gently to evaporate about half of the water, leaving a saturated solution.
5
Leave the solution to cool and crystallise, preferably slowly.
6
Remove the crystals and dry them between filter paper.
Part 3 of 4Why each stage is done
| Stage | Reason |
|---|---|
| Warm the acid | Speeds up the reaction |
| Add the solid in excess | Makes sure all of the acid is used up, so none would become dangerously concentrated on heating |
| Filter | Removes unreacted solid so only salt and water remain |
| Evaporate part of the water | Makes a saturated solution from which crystals can form |
| Cool slowly | Gives larger, better-shaped crystals |
Part 4 of 4Example: copper(II) sulfate
1Reactants
- Reactants Black copper(II) oxide is added to warm, colourless sulfuric acid.
2Reaction
- Reaction CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l); the solution turns blue.
3Filtration
- Filtration Leftover black copper oxide stays on the filter paper.
4Crystallisation
- Crystallisation Blue crystals of hydrated copper(II) sulfate form as the solution cools.
Key terms
| Term | Meaning |
|---|---|
| Soluble salt | A salt that dissolves in water. |
| Excess | More of a reactant than is needed, so some remains unreacted. |
| 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 solution as the solvent evaporates or the solution cools. |
Avoid these mistakes
1
Adding too little base, so unreacted acid is left in the solution that is then heated.
2
Evaporating to dryness; this gives a powder rather than well-formed crystals.
3
Saying the excess solid is filtered off to purify the acid — it is removed to leave the salt solution.
4
Choosing a very reactive metal like sodium to react with acid.
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