One of the principles governing equilibria is Le Chatelier's principle, which states that a dynamic equilibrium acts to counter any change to the equilibrium conditions. An example of this principle is the common ion effect. When salts are dissolved in a solution containing one of the ions from which the salt was formed, the solubility of the salt decreases. Mathematically, under constant conditions, must remain constant. Therefore, if the concentration of one of the ions is increased, the concentration of the other ion must decrease. The only way to decrease the concentration of the other ion (and therefore maintain ) is to form more of the precipitate. Thus, by maintaining , the solubility of the salt has decreased. In other words, when a salt is dissolved in a solvent containing one of the ions common to the salt, is held constant by the formation of more of the precipitate, thus decreasing the concentration of the other ion.
