7.3Acids, Bases and Salts
Preparation of salts — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 10 parts Lesson 3 of 3 1 quiz
Lesson summary
7.3 Preparation of salts — Notes — IGCSE Chemistry notes: Choose and carry out methods for preparing soluble and insoluble salts, using the solubility rules.Salt — an ionic compound formed when the hydrogen of an acid is replaced by a metal ion or ammonium ion.The first part of t…
- Uses of salts
- Which solid
- Excess metal
- Excess insoluble base
- Excess insoluble carbonate
- Conditions
Part 1 of 8Salts and their names
Salt
- Salt an ionic compound formed when the hydrogen of an acid is replaced by a metal ion or ammonium ion.
The first part of the name comes from the metal, oxide or carbonate and the second from the acid.
Hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates.
NaOH + HCl gives sodium chloride, and ZnO + H2SO4 gives zinc sulfate.
Uses of salts
- Uses of salts fertilisers, batteries, cleaning products, healthcare products and fungicides.
Part 2 of 8Solubility rules
| Salt | Soluble | Insoluble |
|---|---|---|
| Sodium, potassium and ammonium salts | All | None |
| Nitrates | All | None |
| Chlorides | Most | Silver and lead(II) |
| Sulfates | Most | Barium, calcium and lead(II) |
| Carbonates | Sodium, potassium, ammonium | Most others |
| Hydroxides | Sodium, potassium, ammonium (calcium partially) | Most others |
Part 3 of 8Choosing the method
| Salt type | Reactants | Method | Separation | Example |
|---|---|---|---|---|
| Soluble (excess solid) | Acid + insoluble base, carbonate or metal | Add excess solid | Filter, crystallise | CuSO4 from CuO |
| Soluble (titration) | Acid + alkali (sodium, potassium or ammonium salts) | Titration with indicator | Repeat without indicator, crystallise | NaCl, KNO3 |
| Insoluble | Two soluble salts | Precipitation | Filter, wash, dry | BaSO4, AgCl |
Decide whether the salt is soluble or insoluble first.
Which solid
- Which solid unreactive metals such as copper do not react with dilute acid, and very reactive metals such as sodium are too dangerous,
- so the oxide or carbonate is used instead.
Sodium salts are soluble, so they are made by titration and not by filtering.
Part 4 of 8Soluble salt from excess solid
Soluble salt from excess solid
Method
Warm a measured volume of dilute acid in a beaker to speed up the reaction.
Add the solid a spatula at a time, stirring, until no more reacts and some solid remains.
Filter off the excess solid; the filtrate is the salt solution.
Heat the filtrate in an evaporating basin until it is saturated (crystals form on a cold glass rod dipped in).
Leave the solution to cool and crystallise, and decant any excess liquid.
Dry the crystals between filter papers or in a warm oven.
The solid, not the acid, must be in excess so that it can be filtered off, and the excess shows that the reaction is complete.
Excess metal
- Mg + H2SO4 → MgSO4 + H2.
Excess insoluble base
- CuO + H2SO4 → CuSO4 + H2O.
Excess insoluble carbonate
- CaCO3 + 2HCl → CaCl2 + CO2 + H2O, stopping when fizzing stops and solid remains.
Part 5 of 8Soluble salt by titration
1Use
- Use for sodium, potassium and ammonium salts made from an alkali and an acid.
2
Method
3
Pipette a measured volume of the alkali into a conical flask and add a few drops of indicator.
4
Add dilute acid from a burette until the colour just changes (the end-point) and record the volume of acid.
5
Repeat with the same volumes of acid and alkali but without indicator, so the salt is not contaminated.
6
Heat the neutral solution in an evaporating basin until concentrated and saturated.
7
Leave to crystallise, decant, then dry the crystals with filter paper.
8Equation
- NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l).
9
Crystallisation gives purer crystals than evaporating to dryness and keeps the water of crystallisation.
10
Do not evaporate to dryness, as the water of crystallisation would be lost.
Part 6 of 8Insoluble salt by precipitation (Supplement)
Insoluble salt by precipitation (Supplement)
Conditions
- Conditions the salt to be made must be insoluble, and the two reactants must be soluble.
Method
Mix a solution containing the metal ion with a solution containing the anion in a beaker, stirring.
A precipitate forms, for example white barium sulfate.
Filter the mixture; the precipitate is the residue.
Wash the residue with distilled water to remove soluble impurities.
Dry the solid in a warm oven or between filter papers.
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq), with ionic form Ag+(aq) + Cl−(aq) → AgCl(s).
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq), with ionic form Ba2+(aq) + SO42−(aq) → BaSO4(s).
Pb(NO3)2(aq) + K2SO4(aq) → PbSO4(s) + 2KNO3(aq).
Ionic equations show only the ions that form the solid, and the other ions are spectators.
Part 7 of 8Hydrated and anhydrous salts
Hydrated salt
Hydrated salt a salt whose crystals contain a definite number of water molecules in the crystal structure.
Water of crystallisation
Water of crystallisation the water molecules chemically combined in the crystal structure of a hydrated salt.
Anhydrous salt
Anhydrous salt a salt that contains no water of crystallisation.
| Feature | Hydrated | Anhydrous |
|---|---|---|
| Copper(II) sulfate formula | CuSO4·5H2O | CuSO4 |
| Colour | Blue | White |
| Made from the other by | Adding water | Heating |
Gentle heating drives off the water of crystallisation, and heating to constant mass ensures that all of the water has been removed.
Removing the water is endothermic, and adding water is highly exothermic.
The change is reversible, CuSO4·5H2O ⇌ CuSO4 + 5H2O.
Cobalt(II) chloride
- Cobalt(II) chloride hydrated CoCl2·6H2O is pink and anhydrous CoCl2 is blue.
Part 8 of 8Reading hydrate formulae and testing for water
Reading hydrate formulae and testing for water
Dot notation
- Dot notation a dot separates the salt formula from its water of crystallisation without changing the salt formula.
CuSO4·5H2O means one formula unit of copper(II) sulfate is associated with five water molecules,
- so one mole of the hydrate contains five moles of water.
Relative formula mass
- Relative formula mass it includes the water, so Mr of CuSO4·5H2O = 63.5 + 32 + 4(16) + 5(18) = 249.5.
Finding the number of water molecules
- Finding the number of water molecules heat the hydrated salt to constant mass and compare the moles of water lost with the moles of anhydrous salt.
Chemical tests for water
- Chemical tests for water anhydrous copper(II) sulfate changes from white to blue and anhydrous cobalt(II) chloride changes from blue to pink when water is added.
Key terms
| Term | Meaning |
|---|---|
| Salt | an ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. |
| Filtrate | the solution that passes through the filter paper. |
| Residue | the solid left on the filter paper. |
| Crystallisation | forming crystals by cooling a saturated solution. |
| Precipitation | forming an insoluble solid when two solutions are mixed. |
| Saturated solution | a solution that cannot dissolve any more solute at that temperature. |
| Hydrated | containing water of crystallisation. |
| Anhydrous | containing no water of crystallisation. |
| Water of crystallisation | water molecules chemically combined in a crystal. |
| Constant mass | a mass that no longer changes on further heating, showing all the water has gone. |
Avoid these mistakes
1
Using the wrong reactant — the solid must be in excess so it can be filtered off, and copper does not react with dilute acid so use copper(II) oxide or carbonate
2
Evaporating all the water — heat until saturated, then cool and crystallise, because drying out loses the water of crystallisation
3
Collecting the filtrate for an insoluble salt — collect the residue, wash with distilled water and dry
4
Crystallising the titration solution with indicator — repeat with the same volumes without indicator
5
Saying a hydrated salt is wet or misreading the 5 in CuSO4·5H2O — the water is part of the crystal, five water molecules for each formula unit
Marked practiceQuizzes for this lesson
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