CHAP. 1: BASIC TERMS [CONTENTS] 31
1.4.4 Reversible and irreversible processes.
The course of any process depends on the conditions under which the given system changes.
If we arrange the conditions in such a way that the system is nearly at equilibrium in every
moment, and that, consequently, the direction of the process may be reversed by even a very
slight change of the initial conditions, the process is calledreversibleor equilibrium. A
reversible process is thus a sequence of (nearly) equilibrium states of a system.
However, processes in the real world are mostly such that the system is out of equilibrium
at least at the beginning. These processes are calledirreversibleornon-equilibrium(the
direction of the process cannot be reversed by any slight change of external conditions, and
the process is a sequence of non-equilibrium states). An equilibrium process is thus actually a
limiting case of a non-equilibrium process going on at an infinitesimal velocity.
Example
Infinitely slow heating or infinitely slow compression of a system may serve as an example of
equilibrium processes which cannot be carried out in practice. In contrast, water boiling at a
temperature of 100◦C and pressure of 101 325 Pa is an example of an equilibrium process which
may take place in practice. If we lower the temperature slightly, the direction of the process will
be reversed and boiling will be replaced by water vapour condensation.1.4.5 Processes at a constant quantity.
In most investigated processes, one or more thermodynamic quantities are maintained constant
during the whole process. These processes are mostly termed using the prefixiso- (is-), and
denoted using the symbol [X], withXindicating the given constant quantity. The following
processes are encountered most often: