2.2.6Chemical Bonding, Application Of Chemical Reactions and Organic Chemistry
Making Soluble Salts
5 min read 6 parts Lesson 19 of 54 1 quiz
Section 1 of 6Choosing reactants
Acid and insoluble base
- Acid and insoluble base A soluble salt can be made by reacting an acid with an insoluble metal oxide, hydroxide or carbonate.
- The solid base does not dissolve, so any left over can be filtered off.
Using a metal
- Using a metal A metal can also be used,
- but only if it lies above hydrogen in the reactivity series and is not so reactive that the reaction is dangerous.
Carbonates
- Carbonates If a carbonate is used, carbon dioxide is also made and escapes as gas, producing fizzing.
Section 2 of 6Method for copper(II) sulfate
1
Warm some dilute sulfuric acid in a beaker.
2
Add copper(II) oxide a little at a time, stirring, until some black solid remains undissolved. This shows the oxide is in excess.
3
Filter the mixture to remove the excess copper(II) oxide, leaving a blue solution of copper(II) sulfate.
4
Heat the filtrate to evaporate about half of the water, until small crystals begin to form.
5
Allow the solution to cool and evaporate slowly. Dry the crystals between filter paper.
Section 3 of 6Equation
| Type | Equation |
|---|---|
| Word equation | copper(II) oxide + sulfuric acid → copper(II) sulfate + water |
| Symbol equation | CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) |
Section 4 of 6Why each step is done
Purity of the salt
Excess solid
- Excess solid Adding more solid than needed ensures that all the acid reacts.
- Unreacted acid would become dangerously concentrated when the solution is evaporated.
Filtering
- Filtering Removing the excess solid leaves only the salt and water in the solution.
Partial evaporation
- Partial evaporation Heating until crystals just begin to appear avoids heating the salt to dryness.
Slow cooling
- Slow cooling Leaving the solution to evaporate over several days gives larger crystals.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Soluble salt | A salt that dissolves in water, so it can be crystallised from solution. |
| Insoluble base | A base that does not dissolve in water, such as copper(II) oxide. |
| Excess | More of a reactant than is needed to react with the other. |
| Filtrate | The liquid that passes through the filter paper. |
| Evaporation | Change from liquid to gas, used to remove water from the solution. |
Section 6 of 6Exam technique
1
Adding the acid in excess instead of the solid — this leaves dangerous acid in the final solution.
2
Evaporating to dryness; stop heating when crystals first appear.
3
Saying the excess solid dissolves — it must be insoluble so that it can be filtered off.
4
Forgetting that a carbonate reaction produces carbon dioxide as well as a salt and water.