Term used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with in static equilibrium will have a constant translational and angular velocity. Dynamic equilibrium occurs whenever a change in the statistical behavior of a large group of particles is balanced by an opposite change in the statistical behavior of a similarly large group of different particles. Dynamic equilibrium applies to one component chemical systems, where the changes are exclusively in state (solid, liquid, gas) or are polymorphic (internal structure). Chemical equilibrium describes chemical systems in which no reactions will occur: two or more phases coexist indefinitely with no changes in their proportions.
Some or all content above used with permission from J. H. Wittke.