Ammonia (NH3) is a colourless gas and liquid with a strong irritating characteristic odour. It is a relatively high-boiling gas with a vapour pressure of 8.7 bar at 20 deg C. Ammonia. is toxic, and will dissolve easily in water. It will form flammable and explosive mixtures with air. Although ammonia itself is toxic, the exhaust gases from the combustion of ammonia and oxygen are not. Ammonia is produced by a Haber-Bosch process, in which the elements, nitrogen and hydrogen, are united at a temperature of 500 to 600 deg C and a. pressure of approximately 200 bar in the presence of a promoted iron catalyst. It is estimated that 4 million tonnes of anhydrous synthetic ammonia were produced in 1959 in the United States, at which time the price of tank-car quantities of refrigeration-grade anhydrous ammonia was $ 80 per tonne.