Final answer:
When the pH of a solution decreases, the acidity increases because a lower pH indicates a higher concentration of hydronium ions, making the solution more acidic.
Step-by-step explanation:
As the pH of a solution decreases (gets lower), the acidity of the solution increases. This occurs because pH is inversely related to the concentration of hydronium ions ([H3O+]) in the solution. Therefore, a decrease in pH indicates an increase in [H3O+], which means the solution is becoming more acidic. A solution with a pH lower than 7 is considered acidic, with 7 being neutral. For instance, if we compare an acidic solution at pH 6.3 with one at pH 7.4, the solution with the pH of 6.3 is more acidic.
It's helpful to remember that the pH scale is logarithmic, so a one-unit change in pH represents a tenfold change in [H3O+]. This means that even a small decrease in pH can indicate a significant increase in acidity. For example, changing the pH from 7 (neutral) to 2 means a 100,000 times increase in the acidity of the solution.