The solution, freed from barium by filtration, may be concentrated by distilling away the water at a low temperature in a vacuum ; the acid remains as a colourless, syrupy liquid, which decomposes at ioo° into perchloric acid, water and chlorine peroxide, C1O₂ ; the last compound is unstable at that temperature, and explodes into chlorine and oxygen. But the peroxide may be prepared by warming, not above 40°, a mixture of potassium chlorate and concentrated sulphuric acid ; the chloric acid decomposes as it is formed: 3HC1O₃ = HC1O₄ + H₀O + 2C1O₂. It is a dark red liquid, boiling at 10.6⁰ to a reddish-brown gas. Unlike nitric peroxide, it does not form double molecules; C1₂O₄ is unknown. It resembles that compound, however, in its action on water ; while nitric peroxide gives a mixture of nitrous and nitric acids, chlorine peroxide, added to an alkali, forms a chlorite and a chlorate : 2C1O₂ + 2KOH.Aq = KC1O₂.Aq + KC1O₃.Aq.

The chlorates, like the nitrates, are all readily soluble in water ; lead and silver chlorites, like the corresponding nitrites, are sparingly soluble salts ; and lead perchlorate is the only salt which doss not easily dissolve. As already mentioned, chloric acid is readily decomposed when its aqueous solution is warmed; chlorous acid is still less stable ; but perchloric acid, which may be prepared by distilling together potassium perchlorate with concentrated sulphuric acid, is relatively stable, seeing that it can be distilled without decomposition. It is an oily liquid, with acid taste ; it is apt to explode when brought into contact with any oxidisable matter. The corresponding perbromic acid is unknown.