A chain reaction is a sequence of reactions where a reactive
product or by-product causes additional reactions to take place. In a
chain reaction, positive feedback leads to a self-amplifying chain of events.
Chain reactions are one way in which systems which are in thermodynamic non-equilibrium can release energy or increase entropy
in order to reach a state of higher entropy. For example, a system may
not be able to reach a lower energy state by releasing energy into the
environment, because it is hindered or prevented in some way from taking
the path that will result in the energy release. If a reaction results
in a small energy release making way for more
energy releases in an expanding chain, then the system will typically
collapse explosively until much or all of the stored energy has been
released. Since chain reactions result in energy transformation into forms associated with larger amounts of entropy. In accordance with the laws of thermodynamics, the reactions cannot be reversed.
A macroscopic metaphor for chain reactions is thus a snowball causing a larger snowball until finally an avalanche results ("snowball effect").
This is a result of stored gravitational potential energy seeking a
path of release over friction. Chemically, the equivalent to a snow
avalanche is a spark causing a forest fire. In nuclear physics, a single
stray neutron can result in an prompt critical event, which may be finally be energetic enough for a nuclear reactor meltdown or (in a bomb) a nuclear explosion.