What is the effect of adding OH- ions to an equilibrium involving H+ ions?

When OH- ions are added to an equilibrium system that includes H+ ions, the OH- ions will react with the H+ ions to form water (H2O). This reaction leads to a decrease in the concentration of H+ ions in the solution. According to Le Chatelier's principle, when you remove a reactant (in this case, H+ ions), the equilibrium will shift in a direction that produces more of that reactant. As a result, the system will respond to the decrease in H+ concentration by shifting the equilibrium to the left, favoring the reactants and thus producing more H+ ions to counteract the change.

Therefore, the correct answer accurately reflects the behavior of the system when OH- ions are introduced, illustrating a fundamental concept in chemical equilibria regarding the response to concentration changes.