What is the common ion effect?

The common ion effect refers to the decrease in solubility of a compound when one of its constituent ions is added to the solution. When a salt is dissolved in water, it dissociates into its ions. If the solution already contains one of those ions due to the addition of a common ion, the solubility equilibrium shifts. According to Le Chatelier's principle, the system will seek to counteract the change by favoring the reverse reaction, thus reducing the solubility of the salt.

This principle is crucial in various applications, such as reducing the solubility of salts in a solution to influence precipitation reactions or to control the pH in buffers. The other options pertain to different chemical concepts that do not accurately describe the common ion effect.