What happens when an ion present in the equilibrium expression is added to a saturated solution?

When an ion present in the equilibrium expression is added to a saturated solution, it disturbs the equilibrium. This occurs due to Le Chatelier's principle, which states that if an external change is applied to a system at equilibrium, the system will adjust to counteract that change.

In the case of a saturated solution, the dissolution and precipitation of the solute are in balance, and the concentration of ions in the solution is at its maximum according to the solubility product constant (Ksp). By adding more of the ion that is already present in the solution, you shift the equilibrium position. Specifically, this can lead to increased precipitation of the solid solute, thereby reducing the amount of dissolved solute in the solution until a new equilibrium is established.

This shift in equilibrium indicates that the system is responding to the increased concentration of that ion by favoring the reverse reaction (precipitation) to bring the concentrations back to the levels dictated by the Ksp. Hence, the addition of the ion effectively disturbs the established equilibrium.