What happens to the hydroxide ion concentration in aqueous solutions when we increase the hydrogen ion concentration by adding an acid?
Would it be good enough to answer by stating that the OH- ion concentration decreases, lowering the pH?
I think, for the coup de grace, I would add because (H^+)(OH^-) = Kw
Yes, stating that the hydroxide ion concentration decreases and the pH is lowered would be a good answer to your question. However, let me explain in more detail how this happens.
When we add an acid to an aqueous solution, it increases the concentration of hydrogen ions (H+). The reason for this is that acids are substances that release hydrogen ions when dissolved in water. The higher concentration of hydrogen ions reduces the availability of hydroxide ions (OH-) in the solution.
Water (H2O) naturally contains both hydrogen ions (H+) and hydroxide ions (OH-), but in equal amounts, resulting in a neutral solution. The pH of a neutral solution is 7.
By adding an acid, the equilibrium between H+ and OH- is disrupted. The higher concentration of H+ ions will react with the OH- ions, forming water molecules (H2O). This reduces the concentration of OH- ions in the solution.
As a result, the hydroxide ion concentration decreases, and the pH of the solution becomes lower than 7. A lower pH value indicates increased acidity. Therefore, increasing the hydrogen ion concentration by adding an acid reduces the hydroxide ion concentration and lowers the pH of the solution.