Perchloric acid, HClO4, is a volatile, unstable, colorless liquid that is a strong, corrosive acid and a powerful oxidizing agent, especially when hot. It explodes if heated to about 90°C or on contact with combustible materials. The monohydrate, HClO4·H2O, is fairly stable and forms needlelike crystals that melt at 50°C. It explodes if heated to 110°C. The dihydrate, HClO4·2H2O, is a stable liquid that boils at 200°C.Formation of Perchlorates
Perchloric anhydride, or chlorine heptoxide, Cl2O7, is a colorless, oily liquid that boils at 82°C without exploding but that may be detonated by shock; it can be prepared by adding phosphorus pentoxide to cold perchloric acid. The perchlorate free radical (chlorine tetroxide, ClO4) can be prepared by adding bromine to silver perchlorate; it is extremely reactive and unstable.
Perchlorates are safer to handle than chlorates; they are more stable when exposed to heat or shock. Potassium perchlorate, KClO4, is perhaps most widely used, e.g., in matches, fireworks, and explosives. It is a colorless crystalline substance that melts at about 610°C.