Colorless, pungent gas composed of nitrogen and hydrogen, chemical formula NH3. Easily liquefied by compression or cooling for use in refrigerating and air-conditioning equipment. Its major use is as a fertilizer, applied directly to soil from tanks of the liquefied gas. Also employed as fertilizers are salts of ammonia, including ammonium phosphate and ammonium nitrate (the latter used in high explosives as well). Ammonia has many other industrial uses as a raw material, catalyst, and alkali. It dissolves readily in water to form ammonium hydroxide, an alkaline solution familiar as a household cleaner.