Anhydrous (water-free) ammonia gas is easily liquefied under pressure (at 20°C liquid ammonia has a vapor pressure of about 120 lb per sq in.) It is extremely soluble in water; one volume of water dissolves about 1,200 volumes of the gas at 0°C (90 grams of ammonia in 100 cc of water), but only about 700 volumes at room temperature and still less at higher temperatures. The solution is alkaline because much of the dissolved ammonia reacts with water, H2O, to form ammonium hydroxide, NH4OH, a weak base. Liquid ammonia is used in the chemical laboratory as a solvent. It is a better solvent for ionic and polar compounds than ethanol, but not as good as water; it is a better solvent for nonpolar covalent compounds than water, but not as good as ethanol. It dissolves alkali metals and barium, calcium, and strontium by forming an unstable blue solution containing the metal ion and free electrons that slowly decomposes, releasing hydrogen and forming the metal amide. Compared to water, liquid ammonia is less likely to release protons (H+ ions), is more likely to take up protons (to form NH4+ ions), and is a stronger reducing agent. Because strong acids react with it, it does not allow strongly acidic solutions, but it dissolves many alkalies to form strongly basic solutions.
Ammonia takes part in many chemical reactions. Ammonia reacts with strong acids to form stable ammonium salts: with hydrogen chloride it forms ammonium chloride; with nitric acid, ammonium nitrate; and with sulfuric acid, ammonium sulfate. Ammonium salts of weak acids are readily decomposed into the acid and ammonia. Ammonium carbonate, (NH3)2CO3·H2O, is a colorless-to-white crystalline solid commonly known as smelling salts; in water solution it is sometimes called aromatic spirits of ammonia. Ammonia reacts with certain metal ions to form complex ions called ammines. Ammonia also reacts with Lewis acids (electron acceptors), e.g., sulfur dioxide or trioxide or boron trifluoride.
Another kind of reaction, commonly called ammonolysis, occurs when one or more of the hydrogen atoms in the ammonia molecule is replaced by some other atom or radical. Chlorine gas, Cl2, reacts directly with ammonia to form monochloramine, NH2Cl, and hydrogen chloride, HCl. Products of such ammonolyses include amides, amines, imides, imines, and nitrides. Ammonia also takes part in oxidation and reduction reactions. It burns in oxygen to form nitrogen gas, N2, and water. In the presence of a catalyst (e.g., platinum) it is oxidized in air to form water and nitric oxide, NO. It reduces hot-metal oxides to the metal (e.g., cupric oxide to copper).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.