In thermodynamics, a component is one of a collection of chemically independent constituents[a][1] of a system.
The latter are combinations of the former, and each species Ai can be represented as a sum of elements: where aij are the integers denoting number of atoms of element Ej in molecule Ai.
[3] If, for example, the species are C (in the form of graphite), CO2 and CO, then Since CO can be expressed as CO = (1/2)C + (1/2)CO2, it is not independent and C and CO can be chosen as the components of the system.
[2][4] Consider an aqueous solution containing sodium chloride (NaCl), potassium chloride (KCl), sodium bromide (NaBr), and potassium bromide (KBr), in equilibrium with their respective solid phases.
While 6 elements are present (H, O, Na, K, Cl, Br), their quantities are not independent due to the following constraints: Consequently, the number of independently variable constituents, and therefore the number of components, is 4.