Only Be and Mg form nitrides directly. 3Be + N2 → Be3N2 3Mg + N2 → Mg3N2 Reaction with hydrogen Alkaline earth metals react with hydrogen to generate saline hydride that are unstable in water. Ca + H2 → CaH2 Reaction with water Ca, Sr and Ba readily react with water to form hydroxide and hydrogen gas. Be and Mg are passivated by an impervious layer of oxide. However, amalgamated magnesium will react with water vapour. Mg + H2O → MgO + H2 Reaction with acidic oxides Alkaline earth metals reduce the nonmetal from its oxide. 2Mg + SiO2 → 2MgO + Si 2Mg + CO2 → 2MgO + C (in solid carbon dioxide) Reaction with acids Mg + 2HCl → MgCl2 + H2 Be + 2HCl → BeCl2 + H2 Reaction with bases Be exhibits amphoteric properties. It dissolves in concentrated sodium hydroxide. Be + NaOH + 2H2O → Na[Be(OH)3] + H2 Reaction with alkyl halides Magnesium reacts with alkyl halides via an insertion reaction to generate Grignard reagents. RX + Mg → RMgX (in anhydrous ether) Identification of alkaline earth cations The flame test The table below presents the colours observed when the flame of a Bunsen burner is exposed to salts of alkaline earth metals. Be and Mg do not impart colour to the flame due to their small size. In solution Mg2+ Disodium phosphate is a very selective reagent for magnesium ions and, in the presence of ammonium salts and ammonia, forms a white precipitate of ammonium magnesium phosphate. Mg2+ + NH3 + Na2HPO4 → (NH4)MgPO4 + 2Na+ Ca2+ Ca2+ forms a white precipitate with ammonium oxalate. Calcium oxalate is insoluble in water, but is soluble in mineral acids. Ca2+ + (COO)2(NH4)2 → (COO)2Ca + NH4+ Sr2+ Strontium ions precipitate with soluble sulphate salts. Sr2+ + Na2SO4 → SrSO4 + 2Na+ All ions of alkaline earth metals form white precipitate with ammonium carbonate in the presence of ammonium chloride and ammonia. Compounds of alkaline earth metals Oxides The alkaline earth metal oxides are formed from the thermal decomposition of the corresponding carbonates. CaCO3 → CaO + CO2 (at approx. 900°C) In laboratory, they are obtained from hydroxides: Mg(OH)2 → MgO + H2O or nitrates: Ca(NO3)2 → CaO + 2NO2 + 1/2O2 The oxides exhibit basic character: they turn phenolphthalein red and litmus, blue. They react with water to form hydroxides in an exothermic reaction. CaO +