ABSTRACT
This chapter provides an introduction to physical and chemical equilibrium. Equilibrium does provide the direction toward which the environment is moving. The study of equilibrium is important and useful for both the understanding of environmental systems, environmental control and treatment systems. Central to the understanding of the equilibrium behavior of systems is assessment of their physical state and the behavior of that state. The environmental engineer has interest in contaminants in any of the three states of matter and the behavior of each of these phases will be discussed in turn. The three states of matter are solid, liquid, and gas. Jacques Charles indicated that a gas increases its volume in direct proportion to temperature in 1787. Boyle subsequently demonstrated that the volume of a gas is inversely proportional to pressure. Both of these relationships are exhibited by the ideal gas law.
