Ammonia (NH3) is a colourless acrid-smelling gas at ambient temperature and pressure. It can be stored and transported as a liquid at a pressure of 10 atm at 25 C. Ammonia dissolves readily in water where it forms, and is in equilibrium with, ammonium ions (NH4+). The sum of ammonia and ammonium concentrations is termed "total ammonia" and, because of the slightly different relative molecular masses, may be expressed as "total ammonia -nitrogen (NH3-N)". In most waters, NH4+ predominates, but increases in pH or temperature or decreases in ionic strength may materially increase levels of non-ionized ammonia.