There are five basic types of salts.
- Normal salts
- Acid salts
- Basic salts
- Double salts
- Complex salts
1. Normal Salts:
Normal salts are formed when all the replaceable hydrogen ions in an acid have been replaced by a metal or Ammonium ion. Example,
In the second reaction, H2SO4 is a dibasic acid, all the hydrogen atoms have been completely replaced.
2. Acid Salts:
An acid salt is formed when the replaceable hydrogen ions [H+] in an acid are only partially replaced by a metal. Example,
\( \scriptsize H_2 SO_{4(aq)} \: + \: NaOH_{(aq)} \: \rightarrow \: \underset{acid \: salt}{NaHSO_{4(aq)}} \: + \: H_2O_{(l)} \)
The above acid salt has all the properties of an acid. The remaining hydrogen ion (H+) can also be replaced by a metal.
\( \scriptsize \underset{acid \: salt}{NaHSO_{4(aq)}} \: + \: \underset{alkali}{NaOH_{(aq)}} \: \rightarrow \: \underset{normal \: salt}{Na_2SO_{4}} \: + \: \underset{water}{H_2O_{(l)}} \)
An acid salt usually results from the reaction between a base and a dibasic acid but not a monobasic acid. The number of acid salts that can be formed from an acid depends on the basicity of that Acid.
Some Acids and their Salts:
| Acid | Metal ion | Acid salt | IUPAC name | Its normal salt |
| H2CO3 | Na+ | NaHCO3 | Sodium hydrogen trioxocarbonate (IV) | Na2CO3 |
| H2SO4 | K+ | KHSO4 | Potassium hydrogen tetraoxosulphate (VI) | K2SO4 |
| H3PO4 | Na+ | NaH2PO4 Na2HPO4 | Sodium dihydrogen tetraoxophosphate (V) Disodium hydrogen tetraoxophosphate (V) | Na2PO3 |
3. Basic Salt:
Basic salt contains hydroxide ion (OH–).
Basic salts are formed when there is an insufficient supply of acid which is needed for the complete neutralization of the base. They have all the properties of base.
Basic salts are formed from metallic ions whose metals have a combining power (or oxidation number) greater than one.
| Basic hydroxide | Metal ion | Ox No | Basic salt | Name | Its normal salt |
| Zn(OH)2 | Zn2+ | +2 | Zn(OH)Cl | Zinc hydroxyl chloride | ZnCl2 |
| mg(OH)2 | Mg2+ | +2 | Mg(OH)Cl | Magnesium hydroxyl chloride | MgCl2 |
| Al(OH)3 | Al3+ | +3 | Al(OH)2NO3 | Aluminium dihydroxytrioxonitrate (V) | Al(NO3)3 |
Basic salts react with acids to form salt and water only.
Examples:
(i) Ba(OH)Cl(aq) + HCl(aq) \( \: \rightarrow \: \) BaCl2 + H2O(l)
(ii) Zn(OH)Br(aq) + HBr(aq) \( \: \rightarrow \: \) ZnBr2 + H2O(l)
4. Double Salts:
These are salts which ionize to produce three different types of ions in solution. Usually, two of these ions are positively charged while the other is negatively charged.
Double salts include alums (with the general formula M+M+++[SO4]2·12H2O) and Tutton’s salts (with the general formula [M+]2M++[SO4]2·6H2O)
Where M+ = Na+, K+ or NH4+ (monovalent ions)
M+++ = Al3+, Fe3+ or Cr3+ (trivalent ions)
Double salts are generally referred to as alum.
Examples:
(i) Ammonium Iron (II) tetraoxosulphate (VI) hexahydrate
(NH2)2 Fe(SO4)2. 6H2O
(ii) Potassium aluminium tetraoxosulphate dodecahydrate K+Al3+(SO4)2. 12H2O. (This is common alum used as a coagulant)
5. Complex Salts:
A complex salt contains a complex ion. Ions in which metal ions form dative bonds with anions or neutral molecules are complex ions.
Examples:
(i) Sodium tetrahydroxozincate (II) – Na2Zn(OH)4
Na2Zn(OH)4(aq) \( \leftrightharpoons \) 2Na+(aq) + [Zn(OH)4]2-
(ii) Potassium hexacyanoferrate(II) – K4Fe(CN)6
K4Fe(CN)6, a complex salt made up of potassium ion, K+ and the complex ion, [Fe(CN)6]4-.
Potassium hexacyanoferrate(II) ionises as follows:
K4Fe(CN)6(aq) \( \leftrightharpoons \) 4K+ + [Fe(CN)6]4-(aq)
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