In the production of ammonia, the following processes occur. 

(a) Air is liquefied at low temperature and high pressure. 

(b) The temperature of the liquid air is raised gradually until the oxygen boils off. Essentially pure liquid nitrogen remains. 

(c) Natural gas is treated with steam to produce carbon dioxide and hydrogen. The proportions of the products vary with the composition of the natural gas. 

(d) Hydrogen and nitrogen (both gases) are combined at high temperature and pressures. They react to form ammonia gas. Some unreacted hydrogen and nitrogen remain. 

(e) The gases are cooled until the ammonia liquefies, and then the gaseous hydrogen and nitrogen are recirculated to react again. 

For each of (a) to (e), indicate whether a physical or chemical change occurs, and briefly explain your choice. For each italicized word or phrase, state whether it refers to an element, a compound, a mixture, or none of these, and briefly explain your choice.