Ammonia (NH₃) is one of the most familiar compounds of nitrogen and hydrogen as well as one of the world's most valuable industrial and agricultural chemicals. Under ordinary conditions, ammonia is a colorless gas that is lighter than air. It has a sharp odor that stings the nostrils and can cause suffocation if inhaled in great enough concentrations. Because of its physical properties, liquefied ammonia has been used as a refrigerant; when the liquid vaporizes, it absorbs a large amount of heat from its surroundings. Ammonia is used throughout the industrial world as a valuable fertilizer. Ammonia and its compounds, primarily ammonium nitrate and other ammonium salts, replenish nitrogen in depleted soils. This greatly increases yields of agricultural crops, especially those that cannot obtain nitrogen from the atmosphere. Some farmers apply anhydrous (dry) ammonia directly to their fields from pressurized tanks. Ammonia is also oxidized to...