Rubidium peroxides

Three peroxides have been prepared by the action of excess of oxygen on the metal. The dioxide, Rb₂O₂, forms yellow needles melting about 600° C., and of density 3.65 at 0° C. Its heat of formation from the elements is about 107.6 Cal. The trioxide, Rb₂O₃, is produced by heating the metal with the theoretical proportion of oxygen, and is a black solid, melting below 500° C., and of density 3.53 at 0° C. It dissolves readily in water, forming hydrogen peroxide, and evolving oxygen. Its heat of formation from the elements is about 125.6 Cal. The tetroxide, Rb₂O₄, is formed as a yellowish-brown substance by the action of oxygen at reduced pressure on the metal, or as a brownish-black mass by the further oxidation of the dioxide. At ordinary pressure in an atmosphere of oxygen it melts between 600° and 650° C. to a dark brown liquid, but in vacuum decomposes at a lower temperature. Its heat of formation from the elements is 137.6 Cal.