Sodium azide is the inorganic compound with the formula . This colorless salt is the gas-forming component in legacy car airbag systems. It is used for the preparation of other azide compounds. It is an ionic substance, is highly soluble in water and is very acutely poisonous. Structure Sodium azide is an ionic solid. Two crystalline forms are known, rhombohedral and hexagonal. Both adopt layered structures. The azide anion is very similar in each form, being centrosymmetric with N–N distances of 1.18 Å. The ion has an octahedral geometry. Each azide is linked to six centers, with three Na–N bonds to each terminal nitrogen center. Preparation The common synthesis method is the "Wislicenus process", which proceeds in two steps in liquid ammonia. In the first step, ammonia is converted to sodium amide by metallic sodium: It is a redox reaction in which metallic sodium gives an electron to a proton of ammonia which is reduced in hydrogen gas. Sodium easily dissolves in liquid ammonia to produce hydrated electrons responsible of the blue color of the resulting liquid. The and ions are produced by this reaction. The sodium amide is subsequently combined with nitrous oxide: These reactions are the basis of the industrial route, which produced about 250 tons per year in 2004, with production increasing due to the increased use of airbags. Laboratory methods Curtius and Thiele developed another production process, where a nitrite ester is converted to sodium azide using hydrazine. This method is suited for laboratory preparation of sodium azide: Alternatively the salt can be obtained by the reaction of sodium nitrate with sodium amide. Chemical reactions Treatment of sodium azide with strong acids gives hydrazoic acid, which is also extremely toxic: Aqueous solutions contain minute amounts of hydrogen azide, the formation of which is described by the following equilibrium: , K = 10−4.6 Sodium azide can be destroyed by treatment with nitrous acid solution: Applications Automobile airbags and aircraft evacuation slides Older airbag formulations contained mixtures of oxidizers and sodium azide and other agents including ignitors and accelerants. An electronic controller detonates this mixture during an automobile crash: The same reaction occurs upon heating the salt to approximately 300 °C. The sodium that is formed is a potential hazard alone and, in automobile airbags, it is converted by reaction with other ingredients, such as potassium nitrate and silica. In the latter case, innocuous sodium silicates are generated.