File:ActivityCoefficientAnimation.gif

English: Animation showing how adding NaCl to a solution can change the enthalpy of a reaction seemingly unrelated to the Na⁺ or Cl^- ions. This is one of many phenomena that can be quantified using the idea of "activity coefficients". (The NaCl changes the "ionic strength" of the solution, which alters the activity coefficients of every ion in the solution, see "Debye–Hückel equation".)

The animation shows a hypothetical reaction A^- + A^- --> A₂^2-, in pure water (left) and salt-water (right). As the two A^- anions approach each other, they experience a stronger and stronger repulsive electric force (orange arrows). However, the repulsion is always much weaker in saltwater than in pure water, because each A^- anion can attract a cloud of Na⁺ cations to screen its charge. Therefore, adding NaCl changes the enthalpy of reaction.