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DOUBLE SALTS AND COORDINATION COMPOUNDS:


When two or more stable compounds combines in a fixed Stoichiometric ratio, the resulting compound is called addition or molecular compounds and they are two type:
(1) DOUBLE SALTS or LATTICE COMPOUNDS:
They are those additions or molecular compounds which looses their identity in aqueous medium i.e. when they dissolved into water, they completely dissociate into their constituents which are simple ions.
Double salts are those molecular compounds which exist only in crystal lattices and lose their identity when dissolved in water. These are formed by mixing two apparently saturated compounds. For example,
(1) When saturated solution of potassium chloride and magnesium chloride is evaporated, we get a new substance called Carnalite.
(2) When saturated solutions of potassium sulphate and aluminum sulphate are mixed and the solution is evaporated, we get the well known compound, Potash alum.
(3) When saturated solutions of ferrous sulphate and al Ammonium sulphate are mixed and the solution is evaporated, we get the well known compound, Mohr’s salt.
Shape and size of the crystals of a double salt are different from that of its component salts. As mentioned above, these compounds exist (are stable) in the solid state and as soon as the lattice is disrupted on dissolution in water or any other solvent or on melting, they decompose into their individual constituents. Thus their solutions will show the same physical and chemical properties as a mixture of solutions of their components. Thus, for example, aqueous solution of potash alum will give the tests of K+1, Al+3 and SO4 2- ions

(2) Coordination or Complex compounds:
Coordination compounds are those molecular compounds which retain their identities even when dissolved in water or any other solvent and their properties are completely different from those of the constituents.

The extent of dissociation of a complex ions into its constituents is decided by a constant which is called formation constant of the complex. The constituents obtained may or may not be detected practically which depends on concentration and the reagents.
 For example, potassium ferrocyanide is a complex compound. It is formed by adding KCN to a saturated solution of ferrous cyanide. If we add a solution of potassium cyanide to a white precipitate of nickel cyanide, Ni (CN)2, the  precipitate immediately dissolves and a red orange solution of a new compound is obtained.
When potassium ferrocyanide is dissolved in water, it does not give the usual tests for Fe2+ and CN ions indicating that these ions which were originally present are not formed when potassium ferrocyanide is dissolved in water,  Actually these ions are present in the form  of a new ion, called ferrocyanide ion which is a complex ion and does not ionize into constituent ions.
Compounds containing complex ions are called complex compounds. Since the complex ions have coordinate1bonds in their structures, these are also known as coordinate ions and hence the corresponding compound as coordinate compound. 
Other common complex ions are nickel cyanide, [Ni(CN)4]-2, cupper ammonium, [Cu(NH3)4]2+  argentocyanide,[Ag(CN)2]+
Thus complex ion may be defined as an electrically charged (cationic or anionic) or even a neutral species and is formed by the combination of a simple cation with more than one neutral molecule or negative ion. For example, ferrocyanide ion is formed by the union of six cyanide ions with ferrous ion; [Ag(NH3)2]+ is formed by the combination of two moles of ammonia and one mole of Ag+ ion. The anions or neutral molecules attached to the central metal atom are called ligands. The central metal cation is generally a transition metal and has a positive 

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