Chloric acid, HClO3·7H2O, is a colorless substance that occurs only in solution. It is a strong acid and a strong oxidizing agent that decomposes if heated above 40°C. Under certain conditions it forms oxygen, water, and the explosive gas chlorine dioxide, ClO2; under other conditions it forms perchloric acid and hydrochloric acid.
A chlorate may be formed (together with the corresponding chloride) by heating the hypochlorite; e.g., 3Ca(ClO)2[right arrow]Ca(ClO3)2+2CaCl2. This reaction takes place when chlorine gas is passed into a hot aqueous solution of a metal hydroxide; the hypochlorite is formed and decomposes almost immediately. Commercially, a chlorate is derived when a hot aqueous metal chloride solution is decomposed by electrolysis, forming chlorine gas at the anode and metal hydroxide at the cathode (with evolution of hydrogen); the chlorine reacts with the hydroxide to form the hypochlorite, which decomposes to form the chlorate.
The most industrially important chlorate is potassium chlorate, or chlorate of potash, KClO3; sodium chlorate, or chlorate of soda, NaClO3, is also used. Potassium chlorate is a colorless crystalline substance that melts at 356°C and decomposes violently at about 400°C. It is a powerful oxidizing agent and is used in making explosives and matches; a mixture of potassium chlorate with phosphorus, sulfur, or any of numerous organic compounds (e.g., charcoal or sugar) explodes upon friction or percussion. When a chlorate is heated, oxygen is evolved, often explosively, and the chloride is formed; e.g., 2KClO3[right arrow]2KCl+3O2. The reaction proceeds controllably at lower temperatures if a catalyst, e.g., manganese dioxide, is used; this provides a convenient source of oxygen. If the chlorate is heated carefully at a lower temperature so that no oxygen is given off, the perchlorate and chloride are formed; e.g., 4KClO3[right arrow]3KClO4+KCl.
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.
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.
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