Caesium monoxide, Cs₂O

The Caesium monoxide, Cs₂O, is prepared by incomplete oxidation of the metal at ordinary temperature, the excess being removed by vacuum-distillation at 180° to 200° C. It forms scarlet-red crystals, which become purple-red at a temperature above the ordinary, and black at 150° C. At -180° C. their colour is bright yellow. The substance melts at 450° to 500° C., decomposing into the metal and the peroxide, Cs₂O₂. At 0° C. it has a density of 4.36 (water at 0° C. = 1). Its heat of formation is 82.7 Cal., and its heat of solution is 83.2 Cal. From the air it absorbs moisture and carbon dioxide, becoming white and deliquescent. It dissolves to a clear solution in water, the process being attended by a hissing sound and the production of flame. In moist carbon dioxide it ignites at the ordinary temperature. Hydrogen reduces it to the hydride and hydroxide.