Double salt
A double salt is a crystalline compound that consists of two different simple salts combined in a fixed stoichiometric ratio. When dissolved in water, a double salt dissociates completely into its constituent ions, just as the individual salts would.
1.0What is Double salt
A double salt is a crystalline compound formed from two simple salts that crystallize together in a unique ionic lattice structure, different from either salt alone. It contains more than one type of cation or anion, as seen in alum, which includes aluminum cations and sulfate anions. Unlike complexes, double salts fully dissociate into their constituent ions in water, behaving like a mixture of the two original salts.
Double salts should not be confused with mixed-crystal systems, where two salts co-crystallize with a fixed composition. Double salts typically have different properties from their individual salt components.
2.0Properties of Double Salts
Crystallization:
- Double salts crystallize from a solution containing the two constituent salts in a specific ratio. They have a well-defined crystalline structure.
Dissociation in Water:
- Upon dissolving in water, a double salt dissociates completely into its individual ions, unlike complex salts (such as coordination compounds) that retain their structure in solution.
- For example, potash alum dissociates into potassium (K+), aluminum (Al3+), and sulfate (SO42−) ions when dissolved in water.
Retention of Individual Salt Properties:
- Double salts retain the properties of the constituent simple salts in their solid form. However, in solution, the properties are typical of the individual ions present.
Non-Complex Nature:
- Unlike coordination compounds, double salts are not considered complexes because they do not have a central metal atom coordinated to ligands. Instead, they are simple mixtures of salts that crystallize together.
Stability:
- Double salts are stable only in their crystalline form. In solution, they break down into their constituent ions, losing the unique properties of the double salt.
3.0Uses of Double Salts
- Water Treatment: Double salts like Alum (Potash alum, KAl(SO4)2⋅12H2O are widely used in water purification processes. Alum acts as a coagulant, helping to remove suspended particles from water.
- Medicine: Some double salts, like Mohr's salt, Ferrous Ammonium Sulfate, FeSO4⋅(NH4)2SO4⋅6H2O have medicinal properties. Mohr's salt is known for its stability and is sometimes used in iron supplements to treat anemia because it provides a stable source of ferrous ions. However, more commonly, alum (potassium aluminum sulfate) has been used in various pharmaceutical formulations for its astringent and antiseptic properties.
- Dyeing and Tanning: In the textile industry, double salts like Potassium Alum (KAl(SO4)2⋅12H2O) are used in dyeing processes and in tanning leather.
- Laboratory Reagents: Double salts like Mohr’s salt are used as reagents in analytical chemistry, particularly in redox titrations due to their stability.
4.0Formation of Double salt
Double salts are formed when two different salts crystallize together from a solution. The resulting compound retains the properties of both original salts and exists as a single crystalline entity. These salts are typically formed by evaporating a solution that contains a mixture of the two simple salts in the correct proportion.
Example: Potash Alum (KAl(SO₄)₂ · 12H₂O)
- Preparation: Dissolve potassium sulfate (K₂SO₄) and aluminum sulfate (Al₂(SO₄)₃) in water in a 1:1 molar ratio.
- Evaporation: Allow the solution to evaporate slowly, leading to the crystallization of potash alum, a double salt.
- Dissociation: In water, potash alum dissociates into K+, Al3+, and SO42−
5.0Difference between Double salts and Coordination compounds
The primary difference between double salts and coordination compounds is how they behave in solution and their structural composition. Double salts dissociate completely into their component ions, losing their identity, while coordination compounds retain their structure and unique properties in both solid and dissolved states. This distinction influences their applications and chemical behavior significantly.
Table of Contents
- 1.0What is Double salt
- 2.0Properties of Double Salts
- 3.0Uses of Double Salts
- 4.0Formation of Double salt
- 5.0Difference between Double salts and Coordination compounds
Frequently Asked Questions
A double salt is a crystalline compound formed by the combination of two different simple salts in a fixed stoichiometric ratio. It retains the properties of both salts in its crystalline form but dissociates completely into its constituent ions in solution.
Unlike single salts, which consist of one type of cation and one type of anion, double salts contain a combination of two different salts crystallized together. Double salts maintain their unique properties only in the solid state and dissociate into their individual ions in solution.
Double salts are formed by dissolving two different salts in water in a specific ratio and then allowing the solution to evaporate slowly. As the solution becomes supersaturated, the salts crystallize together to form a double salt.
When dissolved in water, a double salt dissociates completely into its constituent ions, losing its identity as a double salt. For example, potash alum (KAl(SO4)2⋅12H2O) dissociates into K+, Al3+, and SO42− ions.
Join ALLEN!
(Session 2025 - 26)