∞ generated and posted on 2016.08.22 ∞
A phenomenon that can not only happen but also unhappen, particularly without a requirement for substantial inputs of energy in either direction.
Chemical reactions that are reversible can be described in terms of both forward and reverse reactions, e.g., A → B as well as B → A.
Many chemical reactions that go on inside of cells are reversible and, mostly, those reactions that are not reversible require a substantial input of energy to drive forward. Such often though not always is obtained from the hydrolysis, "burning", of ATP.
All reactions involve some input of energy to happen (or else, in a sense, they already will have happened). With reversible reactions, however, that input of energy, or activation energy, is supplied by the thermal motion, of molecules, that is energy which is already present in a system. In other words, if a reaction is to be reversible then movement of the reaction both forward and in reverse is driven simply by the heat of the environment in which the reactions take place.
The following video shows calcium carbonate decomposition and reformation, including showing what happens when you perturb and equilibrium:
For additional discussion, see [Bioport].