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caustic,any strongly corrosive chemical substance, especially one that attacks organic matter. A caustic alkali is a metal hydroxide, especially that of an alkali metal; caustic soda is sodium hydroxide, and caustic potash is potassium hydroxide. Silver nitrate is another caustic substance; it is sometimes called lunar caustic. Most inorganic acids, e.g., sulfuric acid, are caustic, especially when concentrated.
a medicinal substance that exerts a local cauterizing effect when applied to the skin or a mucous membrane. Caustics are used for the destruction of certain skin neoplasms and for disinfection. They also perform antimicrobial activity by destroying the proteins of microorganisms. Caustics include such acids as fuming nitric acid and trichloracetic acid, such salts of heavy metals as silver nitrate and zinc sulfate, and alcohol solutions of iodine. Caustics are used in small concentrations as astringents.