What happens to the pH of a solution when an acid is added to it?

When an acid is added to a solution, the pH decreases. This occurs because acids release hydrogen ions (H⁺) into the solution. The pH scale measures the concentration of hydrogen ions in a solution, where lower pH values indicate higher concentrations of H⁺ ions and thus more acidic conditions.

For example, in a neutral solution with a pH of 7, the concentration of hydrogen ions is 10⁻⁷ moles per liter. When an acid is introduced, it increases the concentration of H⁺ ions beyond this level, resulting in a pH value that is lower than 7. The extent of the decrease in pH depends on the strength of the acid and the amount added, but the fundamental principle remains that adding an acid will always lower the pH of the solution, confirming the correctness of the answer.