In the potassium .and calcium groups of elements, peroxides are known. When sodium is burned in air a light yellow powder is formed, sodium dioxide, of the formula Na2O2; potassium and caesium yield yellow tetroxides, K2O4 and Cs2O4. Both of these substances react with water, giving off oxygen; but if they are very slowly added to the water, so that the temperature does not rise much, a solution is obtained. The corresponding barium compound is formed when barium monoxide is heated under pressure in air (see p. 13). On addition to water it forms a hydrate, probably Ba=0(OH)2.yH2O. On treatment with acid, hydrogen dioxide, H2O2, is formed; and if sulphuric acid be added in theoretical amount to the barium dioxide, nearly insoluble barium sulphate is formed, along with a fairly pure solution of hydrogen dioxide:
BaO=(OH)2.Aq + H2SO4.Aq = BaSO4+ 0=OH2.Aq. It can be purified, and indeed obtained anhydrous by distillation under very low pressure. It then forms a somewhat viscous, colourless liquid, with a sharp taste.
There is some doubt as to the constitution of hydrogen dioxide, and consequently of the dioxides from which it is derived. It is unlikely that barium ever acts as a tetrad, and much more probable that this character is to be attributed to oxygen; hence the formula of its dioxide is more likely to be Ba=0=0, than 0=Ba=0 ; and consequently hydrogen dioxide has more probably the formula 0=OH2, than HO-OH. Indeed, hydrogen dioxide is possibly a weak acid, since the hydrated dioxides of calcium and barium are precipitated on addition of concentrated solutions of hydrogen dioxide to the hydroxides suspended in water. These substances have all bleaching power, for they readily part with their second atom of oxygen, and it is capable of oxidising coloured insoluble substances to colourless soluble ones.