ABSTRACT
Environmental systems are inherently complex and involve several phases, each containing many components. It is a characteristic of nature, and amply demonstrated through the science of thermodynamics, that when two or more phases are in contact they tend to interact with each other via exchange of matter and/or energy. The phases interact till a state of equilibrium is reached. Two new concepts have to be introduced, namely, fugacity and activity, to describe multi-component heterogeneous systems that display varying degrees of nonideality.
