2.6.5Inorganic Chemistry
Prepare a Soluble Salt — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 32 of 52
Part 1 of 4The principle
Reactants
- Reactants An acid is reacted with an insoluble solid, such as a metal oxide, hydroxide or carbonate, which neutralises it.
- The general equation isacid + insoluble base → salt + water (a carbonate also gives carbon dioxide).
Using a metal
- Using a metal A salt can also be made by reacting an acid with a metal,
- but only a metal above hydrogen that is not so reactive that the reaction becomes dangerous, for example zinc or magnesium.
Example
- Example Copper(II) oxide and dilute sulfuric acid: CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l).
Part 2 of 4Stages of the preparation
1
Warm a measured volume of dilute acid gently in a beaker.
2
Add the solid in small portions, stirring, until no more dissolves and some solid is left at the bottom; this shows the solid is in excess.
3
Filter the mixture to remove the excess solid, collecting the salt solution (the filtrate).
4
Heat the filtrate to evaporate some water, stopping when the solution is saturated and small crystals start to appear, often when about half the water has gone.
5
Leave the concentrated solution to cool and evaporate slowly so that crystals grow.
6
Separate the crystals, then dry them gently between filter paper.
Part 3 of 4Why each choice is made
| Choice | Reason |
|---|---|
| Solid added in excess | Ensures all the acid is used up; leftover acid would become dangerously concentrated on heating and would contaminate the salt. |
| Filtration | Removes the unreacted solid so only salt and water remain in solution. |
| Stop heating when crystals first appear | Boiling to dryness could decompose the salt or spoil the crystal structure. |
| Slow evaporation | Allows large, regular crystals to grow. |
| Warm gently at the start | Speeds up the reaction without risking spitting or boiling. |
Part 4 of 4Alternatives and checks
Making soluble salts
Carbonate base
- Carbonate base Fizzing shows that carbon dioxide is being released;
- the reaction is finished when fizzing stops and solid remains.
Soluble base
- Soluble base If the base is soluble (an alkali) excess cannot be filtered off, so titration is used instead.
Purity
- Purity Rinsing the crystals with a little cold distilled water removes remaining solution without dissolving much salt.
Key terms
| Term | Meaning |
|---|---|
| Soluble | Able to dissolve in a solvent such as water. |
| Insoluble | Unable to dissolve to a significant extent. |
| In excess | More than is needed to react with all of the other reactant. |
| Filtrate | The liquid that passes through the filter paper. |
| Saturated solution | A solution containing as much dissolved solute as it can hold at that temperature. |
Avoid these mistakes
1
Adding the acid in excess instead of the solid. The excess must be the solid, as it can be filtered off.
2
Evaporating all the water. This gives a powder rather than crystals and may decompose a hydrated salt.
3
Saying the filtrate contains the solid. The solid is the residue; the filtrate is the salt solution.
4
Using a very reactive metal such as sodium to make a salt, which is dangerous.
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