Salts: Types, Preparation & Uses

Introduction

Before they are introduced to acids and bases, young chemistry students always think that sodium chloride (common salt) is everything there is to know about salts. However, from their knowledge of acids and bases, they also get to know about other substances, such as copper (II) tetraoxosulphate (VI), potassium trioxocarbonate (IV), ammonium chloride, calcium trioxonitrate (V) etc, which are classified as salts. These substances are the outcomes of the Arrhenius acid-base reactions. So, what is a salt?

Definitions

We will define a salt in terms of basicity (replaceable hydrogen ions) and neutralization.

I) A salt is a substance formed when all or part of the replaceable hydrogen ions in an acid, are replaced by metallic ions (Na+, K+, Mg2+, Ca2+, Cu2+ etc) or ammonium ions (NH4+). This implies that every acid has its corresponding salts. The list below shows examples of some salts and their parent acids.

1. Acid: Hydrochloric acid (HCl)
Salts: Sodium chloride (NaCl), Potassium chloride (KCl), Calcium chloride (CaCl2), Copper (II) chloride (CuCl2) etc

2. Acid: Trioxonitrate (V) acid (HNO3)
Salts: Magnesium trioxonitrate (V) (Mg(NO3)2), Lead trioxonitrate (V) (Pb(NO3)2), sodium trioxonitrate (V) (NaNO3), Silver trioxonitrate (V) etc

3. Acid: Tetraoxosulphate (VI) acid (H2SO4)
Salts: Potassium tetraoxosulphate (VI) (K2SO4), Aluminium tetraoxosulphate (VI) (Al2(SO4)3), Ammonium tetraoxosulphate (VI) ((NH4)2SO4), Copper (II) tetraoxosulphate (VI) (CuSO4) etc

4. Acid: Trioxocarbonate (IV) acid (H2CO3)
Salts: Sodium trioxocarbonate (IV) (Na2CO3), Calcium hydrogen trioxocarbonate (IV), (Ca(HCO3)2), Zinc trioxocarbonate (IV), (ZnCO3), Potassium trioxocarbonate (IV) (K2CO3) etc

II) A salt is a product of the neutralization reaction between an acid and a base.

Examples

a) Hydrochloric acid and sodium hydroxide react to form sodium chloride and water.

HCl(aq) + NaOH(aq) -------> NaCl(aq) + H2O(l)

b) Calcium hydroxide neutralizes tetraoxosulphate (VI) acid to produce calcium tetraoxosulphate (VI) and water.

Ca(OH)2(aq) + H2SO4 (aq) -------> CaSO4(aq) + 2H2O(l)

From the above reactions, we can see that the salts are formed from the exchange of the metallic and nonmetallic radicals of the reactants. In other words, the acids donate the nonmetallic radicals of the salts, while the bases donate the metallic radical components.

Types of Salts

There are five major types of salts, namely: normal, acid, basic, double and complex salts. Let's take a look at each category in details.

Normal Salts: These are salts formed as the products of the complete neutralization of acids by bases. The replaceable hydrogen ions in the acids are completely replaced by metallic or ammonium ions. Examples include sodium chloride, potassium trioxocarbonate (IV), calcium tetraoxosulphate (VI), magnesium trioxonitrate (V), ammonium chloride etc.

Acid Salts: These are salts in which the replaceable hydrogen ions in the acid are partially replaced by metallic or ammonium ions. They are formed when there is an insufficient supply of the base to completely neutralize the acid.

Examples

a) Tetraoxosulphate (VI) acid undergoes incomplete neutralization with potassium hydroxide to form potassium hydrogen tetraoxosulphate (VI) salt.

H2SO4(aq) + KOH(aq) -------> KHSO4(aq) + H2O(l)

On complete neutralization, the product will be potassium tetraoxosulphate (VI) salt

H2SO4(aq) + 2KOH(aq) -------> K2SO4(aq) + 2H2O(l)

From the above equations, it can be observed that for the tetraoxosulphate (VI) acid to be completely neutralized, we will need two moles of potassium hydroxide. Where that is not available as in the first equation, then the reaction will be incomplete and the product formed will be an acid salt. The normal and acid salts ionize in water to produce the tetraoxosulphate (VI) ions and hydrogen tetraoxosulphate (VI) ions respectively as the negative radicals as shown below:

K2SO4(aq) -------> 2K+(aq) + SO4––(aq)

KHSO4(aq) -------> K+(aq) + HSO4–(aq)

b) Sodium hydroxide undergoes incomplete neutralization with trioxocarbonate (IV) acid to form sodium hydrogen trioxocarbonate (IV) salt.

H2CO3(aq) + NaOH(aq) -------> NaHCO3(aq) + H2O(l)

Whereas, complete neutralization produces sodium trioxocarbonate (IV) salt.

H2CO3(aq) + 2NaOH(aq) -------> Na2CO3(aq) + 2H2O(l)

The normal and acid salts ionize in water to produce the trioxocarbonate (IV) ions and the hydrogen trioxocarbonate (IV) ions respectively:

Na2CO3(aq) -------> 2Na+(aq) + CO3––(aq)

NaHCO3(aq) -------> Na+(aq) + HCO3–(aq)

c) Similarly, calcium hydroxide can undergo incomplete neutralization with tetraoxophosphate (V) acid to form more than one acid salts, namely  calcium dihydrogen tetraoxophosphate (V) (Ca(H2PO4)2), and calcium hydrogen tetraoxophosphate (V) (CaHPO4) because it is tribasic.

2H3PO4(aq) + Ca(OH)2(aq) -------> Ca(H2PO4)2(aq) + 2H2O(l)

H3PO4(aq) + Ca(OH)2(aq) -------> CaHPO4(aq) + 2H2O(l)

However, on complete neutralization, the reaction gives a normal salt, calcium tetraoxophosphate (V) salt.

2H3PO4(aq) + 3Ca(OH)2(aq) -------> Ca3(PO4)2(aq) + 6H2O(l)

The two acid salts dissociate in water to produce dihydrogen tetraoxophosphate (V) ions and hydrogen tetraoxophosphate (V) ions respectively.

Ca(H2PO4)2(aq) -------> Ca2+(aq) + 2H2PO4–(aq)

CaHPO4(aq) -------> Ca2+(aq) + HPO4––(aq)

It is important to note that only dibasic and tribasic acids form acid salts because they have more than one replaceable hydrogen ions. This implies that they have the ability to form both normal and acid salts as illustrated in the examples above. Also, the easiest way to theoretically identify acid salts is to look out for  the replaceable hydrogen ion(s) in their molecules.

Basic Salts: Just like the acid salt, a basic salt is produced when there is an insufficient supply of an acid to completely neutralize the base. For a basic salt to be formed, the base must possess at least two hydroxide ions in its molecule.

Examples

a) When an insufficient supply of hydrochloric acid reacts with calcium hydroxide, it forms calcium hydroxide chloride salt.

Ca(OH)2(aq) + HCl(aq) -------> Ca(OH)Cl(aq) + H2O(l)

On complete neutralization, calcium chloride is formed.

Ca(OH)2(aq) + 2HCl(aq) -------> CaCl2(aq) + 2H2O(l)

b) Similarly, trioxonitrate (V) acid can undergo incomplete neutralization with magnesium hydroxide to form magnesium hydroxide trioxonitrate (V) salt.

Mg(OH)2(s) + HNO3(aq) -------> Mg(OH)NO3(s) + H2O(l)

While the complete neutralization reaction produces magnesium trioxonitrate (V)

Mg(OH)2(s) + 2HNO3(aq) -------> Mg(NO3)2(aq) + 2H2O(l)

c) Tetraoxosulphate (VI) acid can undergo incomplete neutralization with aluminium hydroxide to form aluminium hydroxide tetraoxosulphate (VI) salt.

Al(OH)3(s) + H2SO4(aq) -------> Al(OH)SO4(s) + 2H2O(l)

The complete neutralization reaction will produce aluminium tetraoxosulphate (VI) salt.

2Al(OH)3(s) + 3H2SO4(aq) -------> Al2(SO4)3(s) + 6H2O(l)

Double Salts: A salt is said to be a double salt when it ionizes in water to produce three different ions - two positively charged metallic or ammonium ions and one negatively charged nonmetallic ion. They are the products of the crystallization of a mixture of two identical salts in solution, and possess the general formula M+M3+X2-.xH2O because they are hydrated salts.

Examples

a) Aluminium potassium tetraoxosulphate (VI)-dodecahydrate (KAl(SO4)2.12H2O) also known as potash alum, formed from the crystallization of a mixture of equimolar solutions (solutions of equal concentration) of  potassium tetraoxosulphate (VI) and aluminium tetraoxosulphate (VI). The ions present in potash alum are K+, Al3+ and SO4––.

b) Chromium (III) potassium tetraoxosulphate (VI)-dodecahydrate (KCr(SO4)2.12H2O), otherwise called chrome alum, is a product of the crystallization of a mixture of potassium tetraoxosulphate (VI) and chromium (III) tetraoxosulphate (VI) (Cr2(SO4)3). The three ions present in chrome alum are K+, Cr3+ and SO4––.

c) Ammonium iron (II) tetraoxosulphate (VI)-hexahydrate ((NH4)2Fe(SO4)2.6H2O), which is a product of the crystallization of a mixture of equimolar solutions of ammonium tetraoxosulphate (VI) and iron (II) tetraoxosulphate (VI). The ions present in ferrous alum are NH4+, Fe2+ and SO4––

The properties of double salts are the aggregate of those of the two salts they contain.

Complex Salts: These are salts that contain complex ions. Complex ions are those that contain positively charged central ions coordinately bonded to other ions or neutral molecules known as ligands. Examples of complex ions are:

a) tetrahydroxozincate (II) ion [Zn(OH)4]2-, found in sodium tetrahydroxozincate (II) salt (Na2[Zn(OH)4])

Na2[Zn(OH)4](aq) -------> 2Na+(aq) + [Zn(OH)4]2-(aq)

b) hexacyanoferrate (III) ion [Fe(CN)6]3-, present in potassium hexacyanoferrate (III) salt (K3[Fe(CN)6])

K3[Fe(CN)6](aq) -------> 3K+(aq) + [Fe(CN)6]3-(aq)

c) tetrahydroxoaluminate (III) ion [Al(OH)4]-, found in sodium tetrahydroxoaluminate (III) salt (Na[Al(OH)4])

Na[Al(OH)4](aq) -------> Na+(aq) + [Al(OH)4]-(aq)

General Methods of Salts Preparation

The method employed in the preparation of a particular salt in the laboratory depends on whether it is a soluble or an insoluble salt, because of the need to recover it from its solution or the reaction mixture.

I. Preparation of Soluble Salts: The following are the three ways soluble salts can be prepared. These methods are simply the chemical properties of acids, which we studied in Acids & Bases (Part I).

a) Action of Dilute Acids on Active Metals:

acid + active metal --------> salt + hydrogen gas

Examples

Reaction between hydrochloric acid and magnesium ribbons to produce magnesium chloride and hydrogen gas.

2HCl(aq) + Mg(s) -------> MgCl2(aq) + H2(g)

Action of tetraoxosulphate (VI) acid on zinc granules

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

b) Action of Acids on Bases: Acids undergo neutralization reaction with alkalis and insoluble bases to form salts and water.

acid + base -------> salt + water

Examples

Dilute trioxonitrate (V) reacts with potassium hydroxide to form potassium trioxonitrate (V) salt and water.

HNO3(aq) + KOH(aq) -------> KNO3(aq) + H2O(l)

Sodium ethanoate and water are produced when ethanoic acid is neutralized by sodium hydroxide.

CH3COOH(aq) + NaOH(aq) -------> CH3COONa(aq) + H2O(l)

Hydrochloric acid reacts with aqueous ammonia to form ammonium chloride and water.

HCl(aq) + NH4OH(aq) -------> NH4Cl(aq) + H2O(l)

When preparing salts, the reaction of acids with insoluble bases is quite different from that with alkalis. The insoluble base is always added to the acid in a beaker and heated with stirring until the acid reacts completely with it. At that point, the base will no longer dissolve in the acid, and the undissolved portion will be filtered off.

Examples

Heating copper (II) oxide in tetraoxosulphate (VI) acid to produce copper (II) tetraoxosulphate (VI) salt and water.

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

Heating zinc oxide in hydrochloric acid to form zinc chloride and water.

HCl(aq) + ZnO(s) -------> ZnCl2(aq) + H2O(l)

c) Action of Acids on Trioxocarbonate (IV) Salts: Acids react with trioxocarbonate (IV) salts, to form salts (other than trioxocarbonate (IV)) and water.

acid + trioxocarbonate (IV) salt -------> salt + water + carbon (IV) oxide

Examples

Dilute trioxonitrate (V) acid reacts with copper (II) trioxocarbonate (IV) to form copper (II) trioxonitrate (V) salt.

HNO3(aq) + CuCO3(s) -------> Cu(NO3)2(aq) + H2O(l) + CO2(g)

Tetraoxosulphate (VI) acid reacts with calcium trioxocarbonate (IV) to produce calcium tetraoxosulphate (VI) salt.

H2SO4(aq) + CaCO3(s) -------> CaSO4(aq) + H2O(l) + CO2(g)

Recovery of Soluble Salts from Solutions: Depending on their stabilty to heat, soluble salts are recovered from their solutions by evaporation to dryness (e.g, NaCl, K2CO3) or crystallization process (e.g, CuSO4)

II. Preparation of Insoluble Salts: Insoluble salts are salts that do not dissolve in water, whether at room temperature or when heated. They can be prepared in the laboratory by the following methods:

a) Double Decomposition: This is the process of using two soluble substances to produce a soluble and  a volatile or gaseous or an insoluble substance. In the case of salts, it involves the use of two soluble salts to produce soluble and insoluble salts. The insoluble product is separated or recovered from the mixture by filtration.

The choice of the reagents used depends on the insoluble salt you want to prepare. They are selected in a way that each of them will provide one of the radicals needed in the insoluble product. A typical example is the preparation of silver chloride from sodium chloride and silver trioxonitrate (V) salts. The sodium chloride provides the chloride ions, while the silver trioxonitrate (V) provide the silver ions. These two, then, combine to form the silver chloride.

NaCl(aq) + AgNO3(aq) -------> NaNO3(aq) + AgCl(s)

Na+(aq).Cl–(aq) + Ag+(aq).NO3–(aq) ------> Na+(aq).NO3–(aq) + AgCl(s)

Cancel out terms that appear on both sides of the equation

Cl–(aq) + Ag+(aq) -------> AgCl(s)

The silver chloride precipitate is filtered off and dried.

b) Direct Combination of Constituent Elements: Some binary salts are produced by directly combining their constituent elements under appropriate conditions. Example is the combination of metallic iron and sulphur to form iron (II) sulphide.

Fe(s) + S(s) -------> FeS(s)

The following is a list of some soluble and insoluble salts:

I. Soluble Salts

a) All salts of sodium, potassium and ammonium.

b) All trioxonitrate (V) salts.

c) All chloride salts except silver chloride and lead (II) chloride, PbCl2. Lead (II) chloride is however soluble when warmed.

d) All tetraoxosulphate (VI) salts except barium tetraoxosulphate (VI), BaSO4, lead (II) tetraoxosulphate (VI), PbSO4. Calcium tetraoxosulphate (VI) is sparingly soluble.

II. Insoluble Salts

a) All trioxocarbonate (IV) salts, except those of sodium, potassium and ammonium.

b) Silver chloride, barium tetraoxosulphate (VI), lead (II) chloride, and lead (II) tetraoxosulphate (VI).

Uses of Salts

Salts have wide applications in different areas of life such as medicine, agriculture, building & construction, chemical industries etc. Some of them are:

1. Magnesium tetraoxosulphate (VI), also known as Epsom salt, MgSO4.7H2O, is used as laxative.

2. Calcium trioxocarbonate (IV), CaCO3, is used in the manufacture of cement.

3. Calcium tetraoxosulphate (VI)-dihydrate, otherwise called gypsum, CaSO4.2H2O, is used for making Plaster of Paris (POP).

4. Aluminium potassium tetraoxosulphate (VI)-dodecahydrate, called potash alum, KAl(SO4)2.12H2O, is used as a coagulant in water treatment, and in dyeing as a mordant.

5. Sodium chloride, NaCl, is used for cooking, food preservation and glazing of pottery.

6. Potassium trioxonitrate (V), KNO3, is used for making explosives and fertilizers.

Interaction of Substances with Air & Moisture (Efflorescence, Deliquescence & Hygroscopy)

Different substances behave differently when they come in contact with air and moisture.

Efflorescence: This is a phenomenon whereby substances, which possess water of crystallization, lose part of or all their molecules of water of crystallization when exposed to the atmosphere to become anhydrous. It is mainly hydrated salts that undergo efflorescence. Examples are washing soda, Na2CO3.10H2O, blue vitriol, CuSO4.5H2O. These substances are known as efflorescent substances.

Deliquescence: Deliquescent substances are those substances that absorb moisture from the atmosphere and dissolve to become solutions or dilute, if liquid. This phenomenon is known as deliquescence. Examples are sodium hydroxide, potassium hydroxide, calcium chloride, concentrated tetraoxosulphate (VI) acid. They are mostly used as drying agents in the laboratory.

Hygroscopy: This is a phenomenon in which substances absorb moisture from the atmosphere and become sticky, when exposed to air. Such substances as known as hygroscopic substances. Examples include copper (II) oxide, calcium oxide, iron (II) chloride, silica gel etc. They are also used as drying agents in the laboratory.

Do These

Question 1
a) Give two definitions of a salt.
b) Chrome alum is a double salt. Explain what this statement means using the chemical formula of the salt.
c) Give three examples of complex salts with their chemical formulae.

Question 2
a) State four methods of preparing salt with appropriate equations.
b) The basicity of tetraoxophosphate (V) acid is 3. Explain this statement with an equation showing the reaction between the acid and calcium hydroxide.
c) Explain why tetraoxosulphate (VI) acid can form two types of salts, but trioxonitrate (V) acid can form only one type of salt.

Question 3
a) Explain the following with one example each:
i) Deliquescence
ii) Hygroscopy
iii) Efflorescence
b) What substances are used in desiccators?
c) How would you determine experimentally, the water of crystallization of a sample of hydrated copper (II) tetraoxosulphate (VI)? Write the calculation? (To answer this, you may need to read our post on Water of Crystallization)

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