In a acid-base reaction, a salt is formed when a metallic ion or an ammonium ion replaces one or more hydrogen ions in an acid.

E.g. Zinc hydroxide + sulfuric acid zinc sulfate + water

Zn(OH)2 (s) + H2SO4 (aq) ZnSO4 (aq) + H20 (l)

ZnSO4Zn2+ comes from

the base, Zn(OH)2

SO42- comes from

sulfuric acid, H2SO4

Two factors to consider…

Is the salt soluble in water?

Are the starting materials soluble in water?

Soluble Salts Insoluble Salts

All Sodium

All Potassium

All Ammonium

All Nitrates

All Chlorides Lead(II)Silver

All Sulfates BariumCalciumLead

PotassiumSodiumAmmonium

All Carbonates

PotassiumSodiumAmmonium

Hydroxides and Oxides

except

except

except

except

Methods of preparing saltsMethods of preparing salts

Yes No

Yes No

Is the starting material for making the salt soluble in water?

Titration method• Acid + alkali solution• Acid + carbonate solution

Reaction with acids• Acid + excess metal *• Acid + excess insoluble base• Acid + insoluble carbonate

Salt solution Salt crystals (dry with filter paper)

1. Concentrating2. Crystallizing3. Filtering

All insoluble salts can be prepared by precipitation.

Aqueous solutions of two suitable soluble salts are mixed.Eg. Prepare lead(II) sulfate

The insoluble salt appears as a precipitate and can be obtained by filtration.

PbSO4Pb2+ comes from

soluble lead(II) salt

SO42- comes from

sulfuric acid, H2SO4 or any soluble sulfate

Methods of preparing saltsMethods of preparing salts

Yes No

Yes No

Is the starting material for making the salt soluble in water?

Titration method• Acid + alkali solution• Acid + carbonate solution

Reaction with acids with insoluble substances• Acid + excess metal *• Acid + excess insoluble base• Acid + insoluble carbonate

Salt solution Salt crystals (dry with filter paper)

1. Concentrating2. Crystallizing3. Filtering

2. The substance (MCB) must also be insoluble in water.

1. We react the acid with an excess of the substance (MCB).

Reason:This ensures that all the acid is used up. The reaction is complete.

Reason:This allows the excess (unreacted) substance to be filtered from the salt solution produced.

E.g. Prepare zinc sulfate

1. Think of the relevant starting materials.

Zinc + Sulfuric acid Zinc Sulfate + Hydrogen

Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)

ZnSO4Zn2+ comes from

the metal, ZnSO4

2- comes from sulfuric acid, H2SO4

To ensure that the sulfuric acid is completely used up (Why?)

To ensure that the unreacted zinc does not

contaminate the salt formedBy heating to dryness, ZnSO4 will lose

its water of crystallisation

Salts

combine with

water

molecules to

form crystals

This method is suitable for moderately reactive metals such as magnesium, aluminium, zinc, iron and lead.

It is not suitable for reactive metals such as potassium, sodium and calcium. They react violently with acids.

It is not suitable for unreactive metals such as copper and silver. These metals do not react with dilute acids.

PotassiumSodiumCalcium

Magnesium*Aluminium*Zinc*IronLead

CopperSilver

Methods of preparing saltsMethods of preparing salts

Yes No

Yes No

Is the starting material for making the salt soluble in water?

Titration method• Acid + alkali solution• Acid + carbonate solution

Reaction with acids with insoluble substances• Acid + excess metal *• Acid + excess insoluble base• Acid + insoluble carbonate

Salt solution Salt crystals (dry with filter paper)

1. Concentrating2. Crystallizing3. Filtering

E.g. Prepare copper sulfate

1. Think of the relevant starting materials.

Copper(II) oxide + Sulfuric acid Copper (II) Sulfate + Water

CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)

CuSO4Cu2+ comes from

the insoluble baseSO4

2- comes from sulfuric acid, H2SO4

To ensure that the sulfuric acid is

completely used up (Why?)

To ensure that the unreacted CuO does not contaminate the

salt formedBy heating to dryness, CuSO4 will lose its

water of crystallisation.

In the making of copper(II) sulfate, why is copper not used to react with sulfuric acid?

This method is used to prepare metal salts whose metal is not too reactive.

Methods of preparing saltsMethods of preparing salts

Yes No

Yes No

Is the starting material for making the salt soluble in water?

Titration method• Acid + alkali solution• Acid + carbonate solution

Reaction with acids with insoluble substances• Acid + excess metal *• Acid + excess insoluble base• Acid + insoluble carbonate

Salt solution Salt crystals (dry with filter paper)

1. Concentrating2. Crystallizing3. Filtering

In the preparation of soluble salt, what happens if the solid reactant is

soluble in water?

How can we tell if the acid has completely neutralised?

We use the method TITRATION! Use of indicator help identify that the acid

has been completely used up.

Suitable for preparing sodium, potassium, ammonium salts. (Why?)

Find your end point with the help of a suitable indicator then repeat experiment without indicator to get a pure salt sample.