hydrocyanic acid

Hydrocyanic acid, also called prussic acid, is a solution of hydrogen cyanide (HCN) in water; a deadly poisonous weak acid. It is a liquid with an odor of bitter almonds, sometimes used as a fumigant to destroy rats and other pests of buildings. It also has uses in the chemical industry but is so dangerous to handle that its solid salts potassium and sodium cyanide are preferred, even though these, too, are highly poisonous.