Ammonia solutions decrease in density as the concentration of dissolved ammonia increases. Most ammonium salts are soluble and act as acids in liquid ammonia solutions. Ammonotelism is the excretion of ammonia and ammonium ions. Other molecules like hydrogen fluoride, ammonia and methanol can also form hydrogen bonds. Hexamethylenetetramine forms readily from aqueous ammonia and formaldehyde. Several reductants are currently used in SCR applications including anhydrous ammonia, aqueous ammonia or urea. Sulfur sticks are burnt to detect small leaks in industrial ammonia refrigeration systems. Amino acid catabolism results in waste ammonia. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. Underneath, clouds of ammonia and hydrogen sulfide may be found. The soldiers' breath smells like green kale mixed with chlorine and ammonia. It is the product from the reaction of hydrochloric acid and ammonia. Liquid methylamine has solvent properties analogous to those of liquid ammonia. Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. Later process schemes made recycling unused ammonia and carbon dioxide practical.