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A dynamic equilibrium occurs when two reversible processes occur at the same rate. Many processes (such as some chemical reactions) are reversible.

An example of the process can be imagined if a bucket is filled with water and placed in a small room. The water from the bucket will evaporate, and the air in the room will start to become saturated with water vapor. Eventually, the air will be completely saturated with water, and the level of water in the bucket will stop falling. However, water from the bucket is still evaporating. What is happening is that molecules of water in the air will occasionally hit the surface of the water and condense back into the liquid water, and this occurs at the same rate at which water evaporates from the bucket. This is an example of dynamic equilibrium, because the rate of evaporation equals the rate of condensation.

The concept of dynamic equilibrium is not limited to simple changes of state such as that described above. It is often applied to the analysis of chemical reaction kinetics, to obtain useful information about the ratios of reactants and products which will form at equilibrium. It should be noted that at equilibrium the concentrations of the reactants and the concentrations of the products are constant.

The term also has applications across a wide range of disciplines. While it may be applied to less physical systems in these fields, it still relates to a stable situation maintained by balancing processes. For example: in economics it may be used to refer to the constant flux of capital in otherwise stable markets; in ecology, an unchanging population of organisms results from the balancing of birth rate against death rate.

This term can also be used to refer to a steady state (i.e., a state which isn't a true equilibrium, but does not change with time). This can only happen if the system is in contact with an environment which is not in equilibrium.

• Static equilibrium
• Systems theory