Common ion effect occurs when adding an ion already present in the equilibrium.

• A. Adding an ion already present in the equilibrium shifts the reaction to reduce its concentration.
• B. It increases the rate of the reaction without changing concentrations.
• C. It changes the equilibrium constant.
• D. It converts all reactants to products immediately.

Answer: (A)

The idea being tested is Le Chatelier’s principle: adding an ion that is already involved in the equilibrium shifts the balance to oppose that change. When a common ion is introduced, the system adjusts by moving the reaction in the direction that reduces the concentration of that ion in solution. For example, in a dissolution equilibrium like salt ⇌ ions, adding the common ion shifts the balance toward the left (toward the solid or toward consuming the added ion), so the ion’s concentration in solution goes down. This is the essence of the common ion effect—the position of equilibrium changes, but the equilibrium constant itself does not change at a given temperature.

So why is the given statement the best description? It captures that the added ion drives the system to counteract the increase in its own concentration, effectively reducing the ion’s concentration in solution. The other ideas—that the rate always speeds up without changing concentrations, that the equilibrium constant changes, or that all reactants are instantly converted to products—don’t align with how equilibria respond: the constant K stays fixed at a given temperature, and the shift is about position, not an instantaneous, complete conversion.