In thermodynamics, a component
is a chemically distinct constituent of
Calculating the number of components in a system is necessary, for example, when
applying Gibbs phase rule
in determination of the
number of degrees of freedom
of a system.
The number of components is equal to the number of independent chemical
constituents, minus the number of chemical reactions between them, minus
the number of any constraints (like charge neutrality or balance of molar
For example, a system that contains water in liquid state, only, also contains
hydrogen ions and hydroxyl radicals according to the reaction:
- H2O <-> H+ + OH&minus
The number of components in such a system is
- 3 independent chemical constituents &minus 1 chemical reaction &minus 1 constraint (charge neutrality) = 1.
The reactions that are included in the calculations are only those reactions
that actually occur not those that might occur under different conditions
like higher temperature or the presence of a catalyst.
The components of a system are those chemical constituents whose concentration may be varied independently in the various phases.