NCEA Level 3 Chemistry – Aqueous Chemistry (AS91393) Practice Test & Study Guide

Introducing a common ion to a saturated solution of an ionic compound leads to a decrease in the solubility of that compound due to the common ion effect. This phenomenon occurs because the addition of a common ion shifts the equilibrium of the dissolution reaction according to Le Chatelier's principle.

In a saturated solution, the dissolution of the ionic compound is in equilibrium with its undissolved form. When a common ion is added, the increased concentration of that ion disrupts the balance and causes the system to respond by favoring the formation of more undissolved compound. Consequently, this results in precipitation of excess solid and a reduction in the amount of dissolved compound in the solution, hence decreasing the solubility.

The other possibilities do not align with the principles of chemical equilibrium and solubility. For instance, a saturated solution cannot become supersaturated just by the addition of a common ion; rather, it would result in a shift favoring the formation of solid. Additionally, the ionic compound does not completely dissolve, as it is already at saturation. Lastly, while introducing a common ion might affect the pH under certain conditions, this is not a typical observation directly tied to the common ion effect itself.