Preparation of Salts (Soluble & Insoluble)
Choose and describe K324 / 6092 salt-preparation methods: excess insoluble solid, titration or precipitation, with equations, observations and purity reasoning.
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The core idea
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Learning objectives
- describe the techniques used in the preparation, separation and purification of salts as examples of some of the techniques specified in Section 1.2(a) (methods for preparation should include precipitation and titration together with reactions of acids with metals, insoluble bases and insoluble carbonates)
- suggest a method of preparing a given salt from suitable starting materials, given appropriate information.
Salt-preparation questions test a decision, not one recipe. First determine whether the target salt is soluble. Then choose reactants whose physical states allow the product to be separated and purified.
1. Definition
Salt preparation is the selection of suitable reactants and laboratory operations to make, separate and purify a required salt.
| Target and reactants | Method | Product location |
|---|---|---|
| soluble salt from acid + insoluble metal, base or carbonate | add excess solid, filter, crystallise | salt is dissolved in the filtrate before crystallisation |
| soluble salt from acid + alkali or aqueous ammonia | titration, repeat without indicator, crystallise | salt remains in solution before crystallisation |
| insoluble salt from two soluble solutions | precipitation, filter, wash, dry | salt is the residue |
- Apply the solubility rules to the target salt.
- If it is insoluble, use precipitation.
- If it is soluble, ask whether excess reactant can be removed by filtration.
- Use titration when both chosen reactants are solutions.
2. Key Ideas
- Decide whether the target salt is soluble before choosing a method.
- Use excess solid only when the excess reactant can be removed by filtration.
- Use titration when both reactants are solutions, then repeat without indicator.
- Use precipitation for an insoluble salt, then filter, wash and dry the residue.
- A soluble salt is crystallised from the filtrate; an insoluble salt is collected as the residue.
3. Detailed Explanations
A. Soluble salt from an acid and an insoluble solid
Suitable solids include some metals, insoluble metal oxides, insoluble hydroxides and insoluble carbonates. The solid is added in excess so all the acid is used up; any unreacted solid can then be filtered off.
Mark-scheme procedure
- Place the dilute acid in a beaker. For an insoluble base or carbonate, warm it gently if instructed.
- Add the solid a little at a time while stirring.
- Continue until a little solid remains and no further reaction is seen.
- Filter. Discard the excess solid residue; keep the salt solution filtrate.
- Heat the filtrate gently to evaporate some water. Do not heat to dryness.
- Leave the hot concentrated solution to cool so crystals form.
- Filter the crystals and dry them between filter papers.
Example: copper(II) sulfate from copper(II) oxide
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
- Macroscopic observation: black copper(II) oxide disappears and a blue solution forms; excess black solid eventually remains.
- Particle model: ceH + (aq) ions react with oxide ions in the solid. Copper(II) and sulfate ions remain in the filtrate.
- Symbolic representation: the balanced equation includes the solid, aqueous and liquid state symbols.
Other suitable reactions
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)
For a metal or carbonate, effervescence stops when the acid has been used up. If excess solid remains, remove it by filtration. Zinc is not an exception to this separation rule: excess zinc is a solid and can be filtered off.
Use a suitable metal above hydrogen in the reactivity series. Do not choose potassium or sodium because their reactions are dangerously vigorous, or copper and silver because they do not react with dilute hydrochloric or sulfuric acid. Keep flames away when hydrogen is produced.
B. Soluble salt from an acid and an alkali
An acid and an alkali are both aqueous. Adding one in excess would leave a dissolved impurity, which filtration cannot remove. Titration finds the volumes needed for neutralisation.
Mark-scheme procedure
- Use a pipette to transfer a fixed volume of alkali to a conical flask.
- Add a few drops of a suitable indicator.
- Add acid from a burette while swirling; add it dropwise near the end point.
- Record the volume of acid used.
- Repeat using the same measured volumes but without indicator.
- Gently concentrate the pure salt solution, cool it, filter the crystals and dry them.
H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)
The first titration establishes the reacting volumes. Its indicator would contaminate the crystals, so prepare the salt solution again using those volumes without adding indicator.
For detailed burette readings and concordant results, revise Titration Technique.
C. Insoluble salt by precipitation
Choose two soluble solutions that supply the required cation and anion. When mixed, the required ions form an insoluble solid.
Example: barium sulfate
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
- Macroscopic observation: a white precipitate forms.
- Particle model: ceBa²⁺ and ceSO4²⁻ ions join to form a solid lattice; ceNa + and ceCl⁻ remain aqueous spectator ions.
- Symbolic representation: the ionic equation shows only the ions that form the precipitate and balances both atoms and charge.
Mark-scheme procedure
- Mix the two soluble solutions and stir.
- Filter to collect the insoluble salt as the residue.
- Wash the residue with distilled water to remove soluble ions on its surface.
- Dry the product between filter papers.
You do not need to claim that “both solutions are in excess.” The essential planning points are suitable soluble reactants, formation of the precipitate, and correct purification of the residue.
D. Separation and purity language
| Term | Exact meaning | Salt-preparation use |
|---|---|---|
| residue | solid left on the filter paper | excess reactant or insoluble salt product |
| filtrate | liquid passing through the filter paper | soluble salt solution after excess solid is removed |
| wash | rinse a collected solid with a little distilled water | removes soluble impurities from a precipitate |
| concentrate | evaporate some solvent | prepares a soluble salt solution for crystallisation |
| crystallise | form solid crystals from a solution | recovers and purifies a soluble salt |
Heat gently to remove some water, then cool. Strong heating to dryness can cause spitting, product loss or decomposition of some salts.
4. Common Mistakes
- “Soluble salt” describes the target, not necessarily every reactant.
- Filtration cannot remove dissolved acid or alkali.
- In the excess-solid route, the wanted salt is in the filtrate, not the residue.
- In precipitation, the wanted salt is the residue and must be washed before drying.
- Evaporating to complete dryness can lose or decompose product.
5. Exam Tips
Excess solid uses up the acid; filtration removes excess solid; washing removes soluble impurities; concentrating and cooling form crystals.
- Apply the solubility rules before selecting reactants.
- For titration preparation, state that the reacting volumes are repeated without indicator.
- For precipitation, choose two soluble reactants that supply the required ions.
6. Worked Examples
Modelled example 1
choose suitable reactants
Problem
Study the worked solution
Choose acid and insoluble base
Method
Use dilute hydrochloric acid and copper(II) oxide.Reason
Hydrochloric acid supplies chloride while the insoluble oxide supplies copper(II) ions and can be added in removable excess.Working
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l).Ensure the acid is fully used
Method
Add CuO(s) until some solid remains.Reason
Persistent excess solid shows that no acid remains to contaminate the crystals.Working
Warm and add portions until excess CuO is visible.Remove excess solid
Method
Filter the mixture.Reason
Insoluble excess CuO stays as residue while soluble CuCl₂ passes into the filtrate.Working
Keep the copper(II) chloride filtrate.Crystallise and dry
Method
Concentrate gently, cool, filter and dry the crystals.Reason
Evaporation and cooling produce crystals without heating to dryness.Working
Pure CuCl₂ crystals are collected and dried.
Guided practice 2
explain why titration is required
Problem
Use solubility to choose the method
Hints
Hint 1: separation constraint
Hint 2: avoid indicator contamination
View solution step by step
Explain method choice
Method
Use titration rather than an excess reagent.Reason
Both acid and alkali are aqueous, so dissolved excess cannot be filtered away.Working
Titration establishes the neutralising volumes.Prepare clean salt solution
Method
Repeat the exact volumes without indicator.Reason
This avoids both excess reactant and indicator contamination.Working
Obtain neutral KNO₃(aq).Obtain crystals
Method
Concentrate, cool, filter and dry.Reason
Potassium nitrate is soluble, so it must be crystallised from solution.Working
Pure potassium nitrate crystals are collected.
Common misconception 3
correct a separation error
Learner method
Predict what passes through the filter
View solution step by step
Locate the separation error
Method
Reject filtration as a way to remove aqueous sodium hydroxide.Reason
Filter paper separates insoluble solids, not dissolved ions.Working
Excess NaOH(aq) passes into the filtrate.Replace the method
Method
Use titration, repeat without indicator, then crystallise.Reason
Exact neutralising volumes avoid any dissolved excess.Working
Titration → neutral NaCl(aq) → crystals.
Challenge 4
plan a precipitation
Insoluble-product transfer
Keep the correct filter fraction
Hints
Hint 1: form the insoluble pair
Hint 2: purify the solid
View solution step by step
Form the precipitate
Method
Mix calcium nitrate and sodium carbonate solutions.Reason
Mobile Ca²⁺ and CO₃²⁻ ions meet and form insoluble calcium carbonate.Working
Ca(NO₃)₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaNO₃(aq).Separate the product
Method
Filter and retain the residue.Reason
The required CaCO₃ is the insoluble solid; sodium nitrate stays in the filtrate.Working
Residue: white CaCO₃(s).Purify and dry
Method
Wash with distilled water and dry between filter papers.Reason
Washing removes soluble ions and drying removes water.Working
Pure dry calcium carbonate is obtained.
7. Mind Stretchers
Mind stretcher 1: Use the separation constraintExtension
Question: Why is excess copper(II) oxide suitable for making copper(II) sulfate, while excess sodium hydroxide is not suitable for making sodium sulfate?
Show Answer
Excess copper(II) oxide is insoluble and can be filtered off. Excess sodium hydroxide remains dissolved, passes through filter paper and contaminates the salt solution.
Mind stretcher 2: Design suitable reactantsExtension
Question: A student wants lead(II) iodide. Why should both chosen reactants be soluble even though the product is insoluble?
Show Answer
Soluble reactants release the required ions into solution, where Pb²⁺ and I⁻ can meet and form the insoluble precipitate. Soluble spectator ions remain in the filtrate and are washed from the product.
8. Quiz
Test method selection, reagent choice, equations, observations, separation terms and purity reasoning.
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