2.2.7Chemical Bonding, Application Of Chemical Reactions and Organic Chemistry
Preparing Crystals of a Soluble Salt
5 min read 7 parts Lesson 20 of 54 1 quiz
Section 1 of 7What is a salt and why make one?
Definition
- Definition A salt forms when the hydrogen of an acid is replaced by a metal.
- Replacing the H in HCl with potassium, for example, gives potassium chloride, KCl.
Uses
- Uses Salts are used in fertilisers, batteries, cleaning products, healthcare products and fungicides.
Sources
- Sources Some salts are mined, but many have to be prepared in the laboratory from an acid and a suitable solid.
Section 2 of 7Apparatus and reagents
Apparatus
- Apparatus Beaker, measuring cylinder, spatula, stirring rod, filter funnel and paper, evaporating basin, Bunsen burner and indicator paper.
Chemicals
- Chemicals Dilute sulfuric acid, for example 0.5 mol/dm3, and copper(II) oxide as the insoluble base.
Section 3 of 7Method for copper(II) sulfate crystals
1
Measure a volume of dilute sulfuric acid into a beaker and warm it gently with a Bunsen burner.
2
Add copper(II) oxide a little at a time, stirring, until it stops disappearing and a black suspension remains. The oxide is now in excess.
3
Touch the glass rod onto indicator paper to check that the acid has been neutralised.
4
Filter the mixture into an evaporating basin to remove the excess copper(II) oxide.
5
Heat the solution to evaporate roughly half the water so it becomes saturated. Test by dipping in a cold glass rod: crystals forming on it show saturation.
6
Leave the filtrate in a warm place to crystallise.
7
Pour off the remaining solution and dry the crystals by blotting with filter paper.
Section 4 of 7Equation and crystal size
| Item | Detail |
|---|---|
| Word equation | copper(II) oxide + sulfuric acid → copper(II) sulfate + water |
| Symbol equation | CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) |
| Large crystals | Let the filtered solution evaporate slowly over about a week. |
| Small crystals | Evaporate quickly by heating, which gives many tiny crystals. |
| Other salts | The same method works for other soluble salts, such as potassium chloride or potassium sulfate. |
Section 5 of 7Explaining the steps
Typical exam questions
Why use excess base?
- Why use excess base? Excess solid ensures all the acid is used up, so the final product contains no unreacted acid and the solution is neutral.
Why filter?
- Why filter? Filtration removes the unreacted solid so that only the salt solution remains.
Using a carbonate instead
- Using a carbonate instead With copper(II) carbonate the equation is CuCO3(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) + CO2(g),
- and fizzing is seen until the acid is used up.
Safety
- Safety Wear eye protection, avoid heating the solution to dryness, and take care with the hot evaporating basin.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Salt | An ionic compound formed when the hydrogen in an acid is replaced by a metal. |
| Saturated solution | A solution that cannot dissolve any more solute at that temperature. |
| Filtrate | The solution that passes through the filter paper. |
| Excess | More of a reactant than is needed, so some is left over. |
| Crystallisation | Formation of solid crystals from a saturated solution. |
Section 7 of 7Exam technique
1
Not stating that the base is added in excess because that guarantees every bit of acid has reacted.
2
Heating the solution until it is completely dry, which can spoil the crystals.
3
Adding indicator solution to the mixture used to prepare the crystals — in this method only indicator paper is touched with the rod.
4
Forgetting the CO2 product when a carbonate is used.