Preparation Of Salts

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Notes

Naming Salts

A salt is formed when the hydrogen atom in an acid is replaced by a metal or ammonium ion.
The name of a salt has two parts: first from the metal/metal oxide/carbonate, second from the acid.
Hydrochloric acid produces chlorides; sulfuric acid produces sulfates; nitric acid produces nitrates.
Example: sodium hydroxide + hydrochloric acid → sodium chloride.

Method A1: Excess Metal

Used for metals that react with dilute acids (e.g., Mg + H₂SO₄).
Add dilute acid to a beaker, then add metal in small pieces until no more reacts (metal in excess).
Filter to remove excess metal; heat the filtrate gently until concentrated.
Leave to crystallise, decant excess liquid, dry crystals with filter paper.
Example: Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g).

Method A2: Excess Insoluble Base

Used for insoluble bases such as metal oxides (e.g., CuO).
Warm dilute acid gently, add insoluble base slowly while stirring until no more reacts (base in excess).
Filter to remove excess base; heat filtrate until concentrated.
Crystallise, decant, and dry crystals.
Example: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l).

Method A3: Excess Insoluble Carbonate

Used for insoluble metal carbonates (e.g., CaCO₃).
Warm dilute acid, add carbonate slowly until no more reacts (carbonate in excess).
Filter, heat filtrate, crystallise, and dry.
Example: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l).

Method B: Titration (Soluble Alkali)

Used for soluble bases like NaOH.
Pipette alkali into conical flask, add indicator (e.g., phenolphthalein).
Add acid from burette until colour change (end point); record volume.
Repeat without indicator using same volume of acid.
Heat neutral solution to concentrate, crystallise, and dry.
Example: $NaOH(aq) + HCl(aq)$ → NaCl(aq) + H₂O(l).

Preparing Insoluble Salts (Precipitation)

Extended tier only: use precipitation reaction between two soluble reactants.
Mix aqueous solutions of two soluble salts; insoluble salt forms as precipitate.
Filter to collect precipitate, wash with distilled water, dry in warm oven.
Example: Pb(NO₃)₂(aq) + K₂SO₄(aq) → PbSO₄(s) + 2KNO₃(aq).

Hydrated & Anhydrous Salts

Hydrated salts contain water within their crystal structure (e.g., blue CuSO₄·5H₂O).
Anhydrous salts contain no water (e.g., white CuSO₄).
Heating hydrated copper(II) sulfate removes water, turning it white (dehydration).
Adding water to anhydrous copper(II) sulfate reverses the reaction, turning it blue.
Equation: hydrated CuSO₄ ⇌ anhydrous CuSO₄ + water.

States of matter: solid, liquid, gas – particle spacing and arrangement.

Practice questions

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1.Which method is used to prepare a soluble salt when the base is soluble (an alkali)?
Easy
- AExcess metal method
- BExcess insoluble base method
- CExcess insoluble carbonate method
- DTitration method
2.What is the name of the salt formed when zinc oxide reacts with sulfuric acid?
Easy
3.Complete the sentence about naming salts.
Medium
The name of a salt has two parts: the first part comes from $the ____ or$ metal oxide or metal carbonate, and the second part comes from $the ____$ .
4.A salt is a compound formed when the hydrogen atom in an acid is replaced by a metal or ammonium ion.
Easy
True or false?
5.In the preparation of magnesium sulfate crystals, if 0.24 g of magnesium reacts completely with excess sulfuric acid, calculate the maximum mass of magnesium sulfate (MgSO4) that can be obtained. $(Ar: Mg=24, S=32, O=16)$
Medium
- A2.2
- B1.2
- C3.2
- D0.2
6.Arrange the following steps in the correct order for preparing a soluble salt using the excess insoluble base method.
Hard
- Filter the mixture to remove excess base
- Heat the filtrate to concentrate the solution
- Add the insoluble base to warm dilute acid while stirring
- Leave the concentrated solution to crystallize
7.Match each acid to the salt it produces.
Medium
- Hydrochloric acid
- Sulfuric acid
- Nitric acid
- Sulfate
- Chloride
- Nitrate
8.Which of the following is the correct equation for the preparation of calcium chloride using excess calcium carbonate?
Medium
- A $CaCO3 + 2HCl$ → $CaCl2 + CO2 + H2O$
- B $CaCO3 + HCl$ → $CaCl2 + CO2 + H2O$
- C $CaCO3 + 2HCl$ → $CaCl2 + H2 + CO2$
- D $CaCO3 + HCl$ → $CaCl2 + H2O + CO$

Slides

What is a Salt?

A salt is formed when the hydrogen atom in an acid is replaced by a metal or ammonium ion.
For example, replacing H in HCl with potassium gives potassium chloride (KCl).
Salts have many uses including fertilisers, batteries, cleaning products, healthcare products, and fungicides.

Naming Salts

The name of a salt has two parts: the first part comes from the metal, metal oxide, or metal carbonate used; the second part comes from the acid.
For example, hydrochloric acid always produces salts ending in chloride (e.g., sodium chloride).
Zinc oxide reacts with sulfuric acid to produce zinc sulfate.

What is the name of the salt formed when sodium hydroxide reacts with hydrochloric acid?

Sodium chloride
Sodium sulfate
Sodium nitrate
Sodium carbonate

Methods for Preparing Soluble Salts

There are two main methods: Method A (using excess solid reactant) and Method B (using an alkali by titration).
Method A is used when the reactant is a solid in excess; unreacted solid is removed by filtration, then the salt is crystallised.
Method A can be used with excess metal (A1), excess insoluble base (A2), or excess insoluble carbonate (A3).
Method B (titration) is used for soluble bases such as sodium hydroxide.

Method A1: Using Excess Metal

Add dilute acid to a beaker, then add the metal in small pieces while stirring until no more reacts (metal in excess).
Filter to remove excess metal, then heat the filtrate gently until concentrated.
Check saturation by dipping a cold glass rod into the solution; if crystals form, it is saturated.
Leave to crystallise, decant excess liquid, and dry the crystals with filter paper.

In Method A1, why is the metal added in excess?

To ensure all acid reacts
To make the reaction faster
To produce more hydrogen
To prevent crystallisation

Method A2: Using Excess Insoluble Base

Warm dilute acid gently, then add the insoluble base slowly while stirring until no more reacts (base in excess).
Filter to remove excess base, then heat the filtrate gently until concentrated.
Check saturation, leave to crystallise, decant excess liquid, and dry the crystals.

The reaction of copper(II) oxide with sulfuric acid produces copper(II) sulfate $and ____$ .

Method A3: Using Excess Insoluble Carbonate

Warm dilute acid gently, then add the insoluble carbonate slowly while stirring until no more reacts (carbonate in excess).
Filter to remove excess carbonate, then heat the filtrate gently until concentrated.
Leave to crystallise, decant excess liquid, and dry the crystals.

Which gas is produced when calcium carbonate reacts with hydrochloric acid?

Hydrogen
Oxygen
Carbon dioxide
Chlorine

Method B: Using an Alkali (Titration)

Use a pipette to place alkali in a conical flask and add a few drops of indicator (e.g., phenolphthalein).
Fill a burette with acid and add slowly while swirling until the indicator changes colour (end point).
Repeat without indicator using the same volume of acid, then heat the neutral solution to concentrate.
Leave to crystallise, decant excess liquid, and dry the crystals.

Which of these are true about preparing soluble salts?

Method A is used when the base is insoluble.
Method B is used when the base is soluble.
In Method A, the excess solid is removed by evaporation.
In Method B, an indicator is used to find the end point.

Preparing Insoluble Salts (Extended Tier)

Insoluble salts can be prepared using a precipitation reaction.
The solid salt obtained is the precipitate.
Both reactants must be soluble in water, but the product salt must be insoluble.
Method: make aqueous solutions of two soluble salts, mix them, filter to collect the precipitate, wash with distilled water, and dry.

Which of the following is an example of preparing an insoluble salt?

Reacting magnesium with sulfuric acid
Reacting lead(II) nitrate with potassium sulfate
Reacting sodium hydroxide with hydrochloric acid
Reacting calcium carbonate with hydrochloric acid

Hydrated and Anhydrous Salts

Hydrated salts contain water within their crystal structure (e.g., blue copper(II) sulfate).
Anhydrous salts contain no water (e.g., white copper(II) sulfate).
Heating hydrated copper(II) sulfate removes water, turning it white (dehydration).
Adding water to anhydrous copper(II) sulfate reverses the process, turning it blue again.

When hydrated copper(II) sulfate is heated, it turns $____ and$ forms anhydrous copper(II) sulfate.

Which statements about hydrated and anhydrous salts are correct?

Hydrated salts contain water molecules in their structure.
Anhydrous salts are usually coloured.
Adding water to anhydrous copper(II) sulfate turns it blue.
Dehydration is a reversible process.

Summary of Salt Preparation Methods

Soluble salts: use Method A (excess solid) for insoluble bases/metals/carbonates, or Method B (titration) for soluble bases.
Insoluble salts: use precipitation reaction between two soluble salts.
Hydrated salts contain water; anhydrous salts do not. Heating can dehydrate hydrated salts.

Flashcards

What is a salt?

A salt is a compound formed when the hydrogen atom in an acid is replaced by a metal or ammonium ion.

How is a salt named?

The first part comes from the metal, metal oxide, or metal carbonate used; the second part comes from the acid.

What are the two main methods for preparing soluble salts?

Method A: using excess solid reactant (metal, insoluble base, or insoluble carbonate). Method B: using an alkali by titration.

When is Method A (excess solid) used for preparing a soluble salt?

Method A is used when the reactant is a solid in excess; unreacted solid is removed by filtration, then the salt is crystallised.

Describe the steps for preparing a soluble salt using excess metal (Method A1).

Add dilute acid to a beaker, add metal in small pieces while stirring until no more reacts, filter to remove excess metal, heat filtrate to concentrate, leave to crystallise, decant excess liquid, dry crystals with filter paper.

Give an example reaction for preparing a soluble salt using excess metal.

Magnesium + sulfuric acid → magnesium sulfate + hydrogen: Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g).

Describe the steps for preparing a soluble salt using excess insoluble base (Method A2).

Warm dilute acid gently, add insoluble base slowly while stirring until no more reacts, filter to remove excess base, heat filtrate to concentrate, leave to crystallise, decant excess liquid, dry crystals with filter paper.

Give an example reaction for preparing a soluble salt using excess insoluble base.

Copper(II) oxide + sulfuric acid → copper(II) sulfate + water: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l).

Describe the steps for preparing a soluble salt using excess insoluble carbonate (Method A3).

Warm dilute acid gently, add insoluble carbonate slowly while stirring until no more reacts, filter to remove excess carbonate, heat filtrate to concentrate, leave to crystallise, decant excess liquid, dry crystals with filter paper.

Give an example reaction for preparing a soluble salt using excess insoluble carbonate.

Calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l).

When is Method B (titration) used for preparing a soluble salt?

Method B is used when the base is soluble (alkali), e.g., sodium hydroxide.

Describe the steps for preparing a soluble salt by titration (Method B).

Place alkali in conical flask with indicator, fill burette with acid, add acid slowly until indicator changes colour, repeat without indicator using same volume, heat neutral solution to concentrate, leave to crystallise, decant excess liquid, dry crystals.

Give an example reaction for preparing a soluble salt by titration.

Sodium hydroxide + hydrochloric acid → sodium chloride + water: $NaOH(aq) + HCl(aq)$ → NaCl(aq) + H₂O(l).

How do you check if a solution is saturated during crystallisation?

Dip a cold glass rod into the solution; if crystals form on the end, the solution is saturated.

How are insoluble salts prepared?

Insoluble salts are prepared by precipitation: mix aqueous solutions of two soluble salts, filter to obtain the precipitate, wash with distilled water, and dry.

What conditions are needed for preparing an insoluble salt by precipitation?

The solid salt (precipitate) must be insoluble in water, and the reactants must be soluble.

Give an example of preparing an insoluble salt by precipitation.

Lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate: Pb(NO₃)₂(aq) + K₂SO₄(aq) → PbSO₄(s) + 2KNO₃(aq).

What is a hydrated salt?

A hydrated salt is a salt that contains water within its crystal structure.

What is an anhydrous salt?

An anhydrous salt is a salt that contains no water in its structure.

Describe the colour change when hydrated copper(II) sulfate is heated.

Hydrated copper(II) sulfate is blue; upon heating it loses water and becomes white anhydrous copper(II) sulfate.

How can anhydrous copper(II) sulfate be converted back to hydrated copper(II) sulfate?

By adding water to anhydrous copper(II) sulfate, it rehydrates and turns blue again.

Write the reversible equation for the hydration/dehydration of copper(II) sulfate.

Hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water.

What is the purpose of filtering in Method A?

To remove the excess solid reactant (metal, base, or carbonate) that did not react.

Why is the solution heated gently during crystallisation?

To concentrate the solution by evaporating some water, so that crystals form upon cooling.

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