2.3.5Inorganic Chemistry: Part 1
How to Prepare a Soluble Salt — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions
5 min read 6 parts Lesson 14 of 15
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 a solid metal oxide or metal hydroxide that does not dissolve in water.
- The general pattern isacid + metal oxide → salt + water.
Acid plus insoluble carbonate
- Acid plus insoluble carbonate A solid carbonate can be used instead of an oxide.
- The reaction fizzes because carbon dioxide gas is released.
- The general pattern isacid + metal carbonate → salt + water + carbon dioxide.
Acid plus a suitable metal
- Acid plus a suitable metal A metal can also be used,
- but only if it is above hydrogen in the reactivity series,
- so that it displaces hydrogen from the acid.
- Metals that are very reactive, such as sodium or potassium, react dangerously violently and are never used for this.
Which acid gives which salt
- Which acid gives which salt The acid decides the second half of the salt's name: sulfuric acid gives sulfates, hydrochloric acid gives chlorides and nitric acid gives nitrates.
- The metal in the solid decides the first half of the name.
Part 2 of 4Method: copper(II) sulfate crystals
1
Measure a volume of dilute sulfuric acid into a beaker and warm it gently using a water bath or a low Bunsen flame; never let it boil.
2
Add a spatula of black copper(II) oxide powder and stir. The powder dissolves and the solution turns blue.
3
Keep adding the oxide a little at a time until some solid remains at the bottom however long you stir. This shows that the oxide is in excess and all the acid has reacted.
4
Filter the mixture through filter paper in a funnel. The excess copper(II) oxide stays in the paper as the residue; the blue copper(II) sulfate solution passes through as the filtrate.
5
Pour the filtrate into an evaporating basin and heat gently until about half of the water has evaporated, or until small crystals start to appear at the edge of the liquid (the point of crystallisation).
6
Leave the concentrated solution to cool and evaporate slowly for a few days so that large crystals grow.
7
Remove the crystals, pat them dry with filter paper or leave them in a warm place, and store them.
Part 3 of 4Why each stage matters
Making a pure salt
Excess solid
- Excess solid If acid were left over, it would become dangerously concentrated while the solution is heated and would contaminate the crystals.
- Adding solid until no more dissolves guarantees there is no acid left.
Filtration
- Filtration The excess insoluble solid is removed so that only the salt and water remain in solution.
- Nothing else is left in the filtrate.
Gentle evaporation
- Gentle evaporation Heating until about half the water is gone gives a saturated solution.
- Boiling to dryness could spoil the crystals, which contain water and may decompose.
Slow cooling
- Slow cooling Slow crystallisation gives larger, better-shaped crystals than rapid evaporation, which gives a mass of tiny crystals.
Part 4 of 4Equations to learn
| Reactants | Word equation | Symbol equation |
|---|---|---|
| Copper(II) oxide + sulfuric acid | copper(II) oxide + sulfuric acid → copper(II) sulfate + water | CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) |
| Zinc carbonate + hydrochloric acid | zinc carbonate + hydrochloric acid → zinc chloride + water + carbon dioxide | ZnCO3(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l) + CO2(g) |
| Magnesium + sulfuric acid | magnesium + sulfuric acid → magnesium sulfate + hydrogen | Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g) |
Key terms
| Term | Meaning |
|---|---|
| Soluble salt | An ionic compound formed from an acid that dissolves in water, so it can be crystallised from solution. |
| Excess | More of a reactant than is needed, so some is left over when the reaction has finished. |
| Filtrate | The liquid that passes through the filter paper during filtration. |
| Residue | The solid left behind on the filter paper. |
| Crystallisation | The formation of solid crystals from a saturated solution as the solvent evaporates or the solution cools. |
| Saturated solution | A solution that cannot dissolve any more solute at that temperature. |
Avoid these mistakes
1
Saying the solid is added in excess 'to make more salt'. The reason is to make sure all of the acid has reacted so none is left to become concentrated on heating.
2
Evaporating the solution to dryness. Heat only until crystals start to form, then leave the rest to crystallise slowly.
3
Trying to use an alkali such as sodium hydroxide with this method. Excess dissolved alkali cannot be filtered off, so a titration is needed instead.
4
Forgetting that a carbonate gives off carbon dioxide, so the equation must include CO2(g) and the mixture will fizz.
5
Using a very reactive metal such as potassium with acid. It reacts explosively and is unsafe.
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