Double Salt
Definition and meaning of Double Salt in chemistry.
A double salt is a crystalline compound formed when two simple salts combine in a fixed stoichiometric ratio and crystallize together, but which dissociates completely into all of its constituent simple ions when dissolved in water.
In more detail
In the solid state, a double salt has its own distinct crystal lattice and often incorporates water of crystallization, giving it properties different from either parent salt alone. In solution, however, it behaves exactly as a simple mixture of the two salts would, releasing all the same simple ions with no new species formed. This is the key contrast with complex (coordination) salts, whose complex ion survives intact in solution and does not break apart into its simpler components.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| Example formula | KAl(SO4)2·12H2O (potash alum) |
| Behavior in water | Dissociates completely into simple constituent ions |
| Contrast | Complex salts retain an intact coordination ion in solution |
Potash alum, KAl(SO4)2·12H2O, is a double salt of potassium sulfate and aluminum sulfate. When dissolved in water, it dissociates fully into K+, Al3+, and SO4^2- ions, giving the same test reactions as a solution containing K2SO4 and Al2(SO4)3 separately.
Frequently asked questions
How is a double salt different from a complex salt?
A double salt dissociates completely into its simple ions in solution, whereas a complex salt contains a coordination complex ion, such as [Fe(CN)6]^4-, that stays intact and does not break down into simpler ions.
What are some well-known examples of double salts?
Common examples include alums such as potash alum (KAl(SO4)2·12H2O), Mohr's salt or ferrous ammonium sulfate ((NH4)2Fe(SO4)2·6H2O), and carnallite (KCl·MgCl2·6H2O).