activity
/ ak-TIV-ih-tee /
Concentration counts how many ions are in a solution. Activity asks a subtler question: how much do those ions actually 'show up' when it is time to react? In a crowded, salty solution each ion is partly hemmed in by a cloud of oppositely charged neighbours, so it behaves as if it were more dilute than a simple count says. Activity is that effective, behaving concentration.
Formally, activity is concentration multiplied by the activity coefficient. It is the quantity that equilibrium constants, electrode potentials, and pH are truly built from — nature responds to activity, not to the raw head-count of ions. In a very dilute solution, where ions barely feel each other, activity and concentration nearly coincide; in a concentrated one they part ways noticeably.
Activity matters wherever precision matters. The 'true' thermodynamic equilibrium constant is written in activities, and a pH meter actually measures the activity of hydrogen ions, not their concentration. The honest caveat is that activity cannot be measured for a single ion in isolation — only mean values for whole salts are strictly measurable — so single-ion activities and the coefficients behind them are always, to some degree, useful conventions rather than directly weighable facts.
A pH meter reading of 4.0 means the activity of hydrogen ions is 1 x 10^-4, not necessarily their concentration. In a salty sample the actual hydrogen-ion concentration can be measurably higher than that reading implies.
pH measures hydrogen-ion activity, not raw concentration.
By convention the activity of a pure solid or pure liquid is taken as exactly 1, which is why these substances drop out of equilibrium expressions. The activity of a solute is referenced to a standard 1 molal (or 1 molar) state, so it is a dimensionless number.