The common ion effect for weak acids is to significantly decrease the dissociation of the acid in water. Explain the common ion effect.

The common ion effect for weak acids occurs when an ion already involved in the reaction is added to a solution containing the weak acid, leading to a shift in the equilibrium position and reduced dissociation of the acid in water. This phenomenon is a consequence of Le Chatelier's principle, which states that a system in equilibrium will respond to disturbances in a way that counteracts the change. When a common ion is added, it shifts the equilibrium to the left, decreasing the dissociation of the weak acid and resulting in a lower pH than expected for the original solution. This effect is crucial in understanding chemical equilibria and predicting the pH of various systems, such as buffer solutions.