Decreasing the pH of a solution means increasing the concentration of H+ ions protons in the solution. According to Le Chatelier's principle, when a system at equilibrium experiences a change in concentration, temperature, or pressure, the system will shift its equilibrium position to counteract the change. In the case of a weak acid-base reaction, adding more H+ ions decreasing pH will cause the equilibrium to shift to the left, favoring the formation of the weak acid and the conjugate base.For example, consider the weak acid-base reaction between acetic acid CH3COOH and water H2O :CH3COOH + H2O CH3COO- + H3O+In this reaction, acetic acid donates a proton to water, forming the acetate ion CH3COO- and the hydronium ion H3O+ . If we decrease the pH of the solution increase the concentration of H+ ions , the equilibrium will shift to the left, favoring the formation of acetic acid CH3COOH and reducing the concentration of the acetate ion CH3COO- . This shift occurs because the system is trying to counteract the increase in H+ ions by consuming them in the reverse reaction to form more of the weak acid.