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