A simple, quantitative model is suggested to explain the specificity of ions with respect to inhibition of bubble coalescence following a dynamic approach. For the first time, the mode of thinning of the film in between the bubbles, as determined by the density of the bubble dispersion, is recognized as a determining factor. The specificity of the ionic effect is explained by a major difference in adsorption properties of ions, which is enhanced by the film thinning. This leads to charge separation that forms an electrical double layer at each interface of the thin, liquid film, and consequently to electrostatic repulsion. This effect is described by a simple theoretical model that consists of two fundamental equations: mass conservation of each ion in the film, and the Gibbs adsorption equation. In addition, we explain the rapid coalescence of bubbles in purified water under dynamic conditions, which is in contrast with the very slow coalescence under quasi-static conditions.