The addition compounds formed by the combination of two simple salts are termed as double salts.
Ex: FeSO4 (NH4)2 SO4 . 6H2O (Ferrous ammonium sulphate), K2SO4 Al2 (SO4)3 .24H2O (Alum) and other alums.
(ii) Above salts are stable in solid state only.
(iii) When dissolved in water, it furnishes all the ions present in the simple salt from which it has been constituted.
(iv) The solution of double salt shows the properties of the simple salts from which it has been constituted.