7.3Acids, Bases and Salts
Preparation of salts — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 15 parts Lesson 8 of 8 1 quiz
Lesson summary
7.3 Preparation of salts — COMBINED SCIENCE (CHEMISTRY) notes: Describe how soluble and insoluble salts are prepared, separated and purified.Salt — = compound formed when the hydrogen in an acid is replaced by a metal or ammonium ion.Replacing H in HCl with potassium gives KCl.Na…
- Excess solid
- Crystallisation
- Precipitate
- Zinc oxide with sulfuric acid gives zinc sulfate
- Uses — fertilisers, batteries, cleaning products, healthcare products and fungicides
- Warm dilute acid gently in a beaker
Section 1 of 15What a salt is
Salt
- = compound formed when the hydrogen in an acid is replaced by a metal or ammonium ion.
- Replacing H in HCl with potassium gives KCl.
Naming
- Naming the first part comes from the metal, oxide or carbonate and the second part from the acid.
- Hydrochloric acid gives chlorides.
- Zinc oxide with sulfuric acid gives zinc sulfate.
Uses
- Uses fertilisers, batteries, cleaning products, healthcare products and fungicides.
Section 2 of 15Choosing a method
| Salt and base | Method | Separation |
|---|---|---|
| Soluble salt, excess metal | A1: excess solid | Filter, crystallise |
| Soluble salt, insoluble base | A2: excess solid | Filter, crystallise |
| Soluble salt, insoluble carbonate | A3: excess solid | Filter, crystallise |
| Soluble salt, soluble alkali | B: titration | Repeat without indicator, crystallise |
| Insoluble salt | Precipitation | Filter, wash, dry |
Section 3 of 15Method A: excess solid reactant
1
Warm dilute acid gently in a beaker.
2
Add the metal, insoluble base or insoluble carbonate in small amounts, stirring, until no more reacts.
3
Filter to remove the excess solid.
4
Heat the filtrate in an evaporating basin until the solution is concentrated.
5
Check it is saturated by dipping a cold glass rod in; crystals form on it.
6
Leave to crystallise, decant extra liquid and dry the crystals with filter paper.
Excess solid
- Excess solid shows that all the acid has reacted.
Examples
- Examples magnesium sulfate from magnesium and sulfuric acid;
- copper(II) sulfate from copper(II) oxide;
- calcium chloride from calcium carbonate.
Section 4 of 15Method B: titration with an alkali
1
Use a pipette to put alkali in a conical flask and add indicator.
2
Add acid from the burette until the indicator changes colour and record the volume.
3
Repeat with the same volumes without indicator.
4
Evaporate the neutral solution to concentrate it and test for saturation.
5
Leave to crystallise and dry the crystals.
- Use
- for soluble bases such as sodium hydroxide, giving sodium chloride from sodium hydroxide and hydrochloric acid.
Section 5 of 15Insoluble salts by precipitation (supplement)
1
Make solutions of two soluble salts.
2
Mix them with a stirring rod.
3
Filter to collect the precipitate.
4
Wash it with distilled water.
5
Leave it to dry in a warm oven.
Condition
- Condition the salt formed must be insoluble and the reactants must be soluble.
Example
- Example lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate.
Section 6 of 15Solubility rules
| Salts | Soluble | Insoluble |
|---|---|---|
| Sodium, potassium, ammonium | All | None |
| Nitrates | All | None |
| Chlorides | Most | Silver, lead(II) |
| Sulfates | Most | Barium, calcium, lead(II) |
| Carbonates | Sodium, potassium, ammonium | Most others |
| Hydroxides | Sodium, potassium, ammonium (calcium sparingly) | Most others |
Section 7 of 15More from the specification
1Hydrated vs anhydrous salts
- Hydrated salts contain chemically bonded water of crystallisation within their crystal structure,
- (e.g. blue CuSO4.5H2O)
- whereas anhydrous salts lack water (white CuSO4).
2Insoluble salt preparation (Precipitation)
- Mixing two soluble salt solutions causes insoluble solid precipitate to form, followed by filtration, washing with distilled water, and drying.
3Solubility rules
- All nitrates, group 1 salts, and ammonium salts are soluble;
- most sulfates are soluble except barium, lead, and calcium.
4Excess insoluble base method
- Adding excess insoluble metal oxide or carbonate to warm acid, filtering off unreacted solid, and crystallising the filtrate.
5Hydrated
- Containing water of crystallisation.
6Anhydrous
- Containing no water of crystallisation.
7Crystallisation
- Obtaining a dissolved solid from a solution by evaporating some of the solvent and cooling.
8Solubility
- Mass of solute that dissolves in a given mass of solvent.
9Mixture
- Two or more substances mixed but not chemically combined.
10Anhydrous salt
- A salt with no water of crystallisation.
Section 8 of 15Hydrated and anhydrous salts
Hydrated salt
- Hydrated salt contains water molecules in its crystal structure.
Water of crystallisation
- = the water incorporated in the crystal structure.
Anhydrous salt
- Anhydrous salt contains no water of crystallisation.
Section 9 of 15Copper(II) sulfate
Hydrated
- Hydrated blue crystals.
Anhydrous
- Anhydrous white powder.
Heating
- Heating gentle heating removes the water of crystallisation and the blue solid becomes white.
Adding water
- Adding water restores the blue colour, so the change is reversible.
Section 10 of 15Reading a hydrate formula
Dot
- Dot a dot separates the salt from its water of crystallisation.
CuSO4·5H2O
- CuSO4·5H2O one formula unit of copper(II) sulfate has five water molecules, so one mole of hydrate contains five moles of water.
CoCl2·6H2O
- CoCl2·6H2O is hydrated cobalt(II) chloride.
Anhydrous forms
- CuSO4 and CoCl2.
Section 11 of 15Reversible equations
| Change | Equation |
|---|---|
| Dehydration by heating | CuSO4·5H2O → CuSO4 + 5H2O (blue to white) |
| Rehydration by adding water | CuSO4 + 5H2O → CuSO4·5H2O (white to blue) |
Section 12 of 15Hydrates in salt preparation
Crystallisation
- Crystallisation some water can be retained in the crystals when a salt is crystallised from solution.
Drying
- Drying drying crystals gently avoids removing the water of crystallisation.
Section 13 of 15More from the specification
1Preparing Soluble Salts with Excess Base
- An insoluble metal, metal oxide,
- or carbonate is added in excess to dilute acid to ensure all acid is consumed, preventing contamination of the salt.
2Filtration and Evaporating Basin Crystallisation
- The unreacted excess insoluble solid is removed by filtration,
- and the clear filtrate is gently heated in an evaporating basin to saturation point.
3Salt Preparation by Titration
- Soluble salts of reactive alkali metals like sodium and potassium cannot be made using excess solids and must be prepared by acid-base titration.
4Precipitation of Insoluble Salts
- Insoluble salts are synthesised by mixing two aqueous soluble salt solutions, filtering off the solid precipitate, washing with distilled water,
- and drying.
5Preparing Soluble Salts with Excess Base
- An insoluble metal, metal oxide,
- or carbonate is added in excess to dilute acid to ensure all acid is consumed, preventing contamination of the salt.
6Filtration and Evaporating Basin Crystallisation
- The unreacted excess insoluble solid is removed by filtration,
- and the clear filtrate is gently heated in an evaporating basin to saturation point.
7Salt Preparation by Titration
- Soluble salts of reactive alkali metals like sodium and potassium cannot be made using excess solids and must be prepared by acid-base titration.
8Precipitation of Insoluble Salts
- Insoluble salts are synthesised by mixing two aqueous soluble salt solutions, filtering off the solid precipitate, washing with distilled water,
- and drying.
9Hydroxide
- A compound containing the OH− ion, formed by Group I metals with water.
10Titration
- Method for finding the volume of one solution that reacts exactly with another.
11Mixture
- Two or more substances mixed but not chemically combined.
12Evaporating basin
- A dish used to evaporate solutions.
13Sodium chloride
- Compound of sodium and chlorine; common salt.
14Excess
- More than enough of a reactant.
15Insoluble
- Unable to dissolve.
16Soluble
- Able to dissolve in a solvent.
17Carbonate
- A compound containing the CO3 group, such as calcium carbonate.
18Precipitate
- An insoluble solid formed in a solution.
19Solubility
- Mass of solute that dissolves in a given mass of solvent.
20Alkali
- A soluble base that produces OH− ions in water.
Section 14 of 15Key terms
| Term | Meaning |
|---|---|
| Salt | Compound formed when an acid's hydrogen is replaced by a metal or ammonium. |
| Excess | More than needed so that all the acid reacts. |
| Filtrate | Liquid that passes through filter paper. |
| Crystallisation | Formation of crystals from a saturated solution. |
| Precipitate | Insoluble solid formed in a reaction. |
| Titration | Measuring volumes that react exactly. |
| Term | Meaning |
|---|---|
| Hydrated salt | Salt that contains water of crystallisation. |
| Anhydrous | Containing no water of crystallisation. |
| Water of crystallisation | Water molecules in the crystal structure. |
| Dehydration | Removal of water by heating. |
| Rehydration | Gain of water by an anhydrous salt. |
Section 15 of 15Exam technique
1
Evaporating all the water — crystals are obtained by cooling a saturated solution.
2
Using titration for insoluble bases — use excess solid.
3
Forgetting to filter off the excess solid.
4
Using an insoluble reactant in a precipitation preparation.
1
Writing the hydrate formula without the dot or the number of water molecules.
2
Saying the anhydrous salt contains water.
3
Saying dehydration is irreversible.
4
Confusing blue (hydrated) and white (anhydrous) copper(II) sulfate.
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