Chemically reacting systems with three independent State functions are systematically examined with a mathematical tool with some differences from the Usual one. The thermodynamic relations for the usual fluids depending upon MO State functions are obtained as a Particular case in the absence of the chemical state function. Fluids which obey the relation (V = K(V)f(p/T)). formally similar to Curie's law for magnetic systems, are also investigated. Quasi-static irreversible processes with Uniform intensive properties are considered.