Most metal hydroxides other than those of group I metals and barium, strontium and ammonia are insoluble. The nature of these insoluble hydroxides often gives useful information regarding the nature of the metal. Transition metal hydroxides often have typically characteristic colours while amphoteric metal hydroxides tend to form soluble salts in excess hydroxide. Finally some metal ions form soluble complexes with ammnonia and so tend to dissolve in excess ammonium hydroxide solution. Although not metallic, the ammonium ion is cationic and is thus dealt with in this category.
Ammonium salts give off ammonia when boiled with aqueous sodium hydroxide.
The presence of ammonia may be demonstrated by smell or with moist red litmus paper - which turns blue. These are best detected by flame test. This is also true for the group II metal barium which does not give a precipitate with aqueous sodium hydroxide. The white precipitate formed is not soluble in excess aqueous sodium hydroxide. The two metals may be distinguished by adding dilute sulphuric acid to a fresh solution of the compound, as a white precipitate of calcium sulphate forms if the metal is calcium but no precipitate forms with magnesium salts.