Fifth group of cations magnesium, sodium, potassium, and ammonium

34 FIFTH GROUP OF CATIONS MAGNESIUM, SODIUM, POTASSIUM, AND AMMONIUM. 

Group reagent there is no common reagent for the cations of this group. Group reaction cations of the fifth group do not react with hydrochloric acid, hydrogen sulphide, ammonium sulphide or (in the presence of ammonium salts) with ammonium carbonate. Special reactions or flame tests can be used for their identification. 

The reactions of ammonium ions are quite similar to those of the potassium ion, because the ionic radii of these two ions are almost identical. 

Magnesium hydroxide, if ammonium salts are absent, is practically insoluble. Magnesium dissolves readily in acids  

Magnesium forms the bivalent cation Mg2+. Its oxide, hydroxide, carbonate, and phosphate are insoluble the other salts are soluble. They taste bitter. Some of the salts are hygroscopic. 

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