【How-to】Which polyatomic ions are always soluble

All compounds of Alkali metal (Group 1, or, Group IA) cations, are soluble. … EXCEPT those of ammonium (NH₄⁺) and Alkali metal (Group 1, or, Group IA) cations. All hydroxides are insoluble. EXCEPT those of ammonium (NH₄⁺), barium and alkali metal (Group 1, or, Group IA) cations.