Here's why:

* pH scale: The pH scale measures the acidity or alkalinity of a solution. It ranges from 0 to 14.

* Acidity: Acids have a higher concentration of hydrogen ions (H+).

* pH and H+ concentration: The pH scale is logarithmic, meaning each decrease in pH by one unit represents a tenfold increase in the concentration of hydrogen ions.

Therefore, as a solution becomes more acidic (more H+ ions), the pH value decreases.