Activity
The effective concentration of a species, corrected for non-ideal interactions with its surroundings.
Formula / equation
a = γ c / c°
Unit
Dimensionless
In depth
Activity replaces concentration in rigorous thermodynamics because ions in real solutions are shielded by their oppositely charged neighbours and do not behave independently. In very dilute solution activity approaches concentration, but in concentrated electrolytes the difference is substantial.
Examples in the real world
In 0.1 M NaCl the activity of Na⁺ is only about 0.078, some 22% below its molar concentration.