Double salts are crystalline molecular or addition compounds containing more than one salt in simple molecular proportions soluble in water and in solution they ionise and exhibit all the properties of the constituent ions.
For example,
K2SO4 +Al2(SO4)3 - 24H2O
K2SO4 . Al2(SO4)3 . 24H2O(aq) ⟶ 2K+(aq) + 2Al3+(aq) + 4SO2-4(aq) + 24H2O
