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In thermodynamics, a component is a chemically-independent constituent of a system. The number of components represents the minimum number of independent species necessary to define the composition of all phases of the system.
The number of components is equal to the number of distinct chemical species (constituents), minus the number of chemical reactions between them, minus the number of any constraints (like charge neutrality or balance of molar quantities).
A system that contains water in the liquid state also contains hydronium cations and hydroxyl anions according to the reaction:
- 2 H2O H3O+ + OH-
The number of components in such a system is
- 3 chemical constituents - 1 chemical reaction - 1 constraint (charge neutrality) = 1.
CaCO3 - CaO - CO2 system
This is an example of a system with several phases, which at ordinary temperatures are two solids and a gas. There are three chemical species (CaCO3, CaO and CO2) and one reaction CaCO3 CaO + CO2. The number of components is then 3 - 1 = 2.
If the composition of the same system is instead expressed in terms of ions, the number of independent components does not change. There are now 4 chemical species (Ca2+, CO32-, O2- and CO2) and the number of components is calculated as
- 4 chemical constituents - 1 chemical reaction - 1 constraint (charge neutrality) = 2.
Water - Hydrogen - Oxygen
The reactions included in the calculation are only those that actually occur under the given conditions, and not those that might occur under different conditions such as higher temperature or the presence of a catalyst. For example, the dissociation of water into its elements does not occur at ordinary temperature, so a system of water, hydrogen and oxygen at 25 °C has 3 independent components.
- Peter Atkins and Julio de Paula, "Physical Chemistry" 8th Edition (W.H. Freeman 2006), p.175-176