Double salt
Encyclopedia
Double salts are salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

s containing more than one cation or anion. They form when more than one salt is dissolved in a liquid and when together they crystallize
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...

 in a regular pattern. A well-known double salt is alum
Alum
Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate with the formula KAl2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB2.12H2O.-Chemical properties:Alums are...

 containing two cations (potassium and aluminium) and a sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

 anion. Other examples are potassium sodium tartrate
Potassium sodium tartrate
Potassium sodium tartrate is a double salt first prepared by an apothecary, Pierre Seignette, of La Rochelle, France. As a result the salt was known as Seignette's salt or Rochelle salt....

 and bromlite.

Alums are double sulphates of a monovalent cation and a trivalent cation containing 24 water of crystallization.

Difference between a double salt and a complex...
Double salts when dissolved in water dissociate into simple ions completely while complexes do not dissociate into simple ions and maintain the property of complexes.

"A double salt is a substance obtained by the combination of two different salts which crystallise together as a single substance but ionize as two distinct salts when dissolved in water."

When a double salt is formed by the chemical combination of two single salts (like, potassium tartrate and sodium tartrate), the properties of the double salt formed will not be the same as the properties of its component single salts.
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