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Physical And Chemical Properties Of Ammonia

Ammonia (boiling point -28.03°F [-33.35°C]) can be made in the laboratory by heating ammonium chloride with lime, and the gas collected by downward displacement of air, or displacement of mercury. Water solutions of ammonia, called ammonium hydroxides, having as much as 28% ammonia by weight, can be obtained by this method. Ammonium hydroxide exhibits the characteristics of a weak base, turning litmus paper blue, and neutralizing acids with the formation of ammonium salts. Transition metal ions are either precipitated as hydroxides (iron [II], iron [III]) or converted to ammonia complexes (copper [II], nickel [II], zinc [II], silver [I]). The copper (II) ammonia complex, in solution, is deep blue in color, and serves as a qualitative test for copper. It also has the ability to dissolve cellulose, and has been used in the process for making regenerated cellulose fibers, or rayon.

Ammonia molecules possess a pyramidal shape, with the nitrogen atom at the vertex. These molecules continually undergo a type of motion called inversion, in which the nitrogen atom passes through the plane of the three hydrogen atoms like an umbrella turning inside out in the wind. When ammonia acts as a base, the nitrogen atom bonds either to a proton (to form ammonium ion) or to a metal cation. Ammonium salts such as ammonium chloride, called sal ammoniac, are water soluble and volatile when heated. It is often found that considerable heat is absorbed when ammonium salts dissolve in water, leading to dramatic reduction in temperature. Ammonium salts containing anions of weak acids (carbonate, sulfide) easily liberate ammonia owing to the tendency of a proton to break off the nitrogen atom and be bound by the weak acid anion.

In liquid or frozen ammonia, the molecules attract one another through sharing a hydrogen atom between one molecule and the next, called hydrogen bonding. In this attraction, called association, compounds apparently containing free electrons can be obtained by treating sodium/ammonia solutions with complexing agents.

Ammonia is a flammable gas, and reacts with oxygen to form nitrogen and water, or nitrogen (II) oxide and water. Oxidation of ammonia in solution leads to hydrazine, a corrosive and volatile ingredient in fuels. Ammonium salts of oxidizing anions—nitrate, dichromate, and perchlorate—are unstable and can explode or deflagrate when heated. Ammonium nitrate is used as a high explosive; ammonium perchlorate as a component of rocket fuels. Ammonium dichromate is used in a popular artificial volcano demonstration, in which a conical pile of the salt is ignited and burns vigorously, throwing off quantities of green chromium (III) oxide-the lava.

When ammonium hydroxide is treated with iodine crystals, an explosive brown solid, nitrogen triiodide, is formed. When dry, this substance is so sensitive that the lightest touch will cause it to explode with a crackling sound and a puff of purple iodine vapor.

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