Here's why:

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

* 0-6.9: Acidic (high concentration of H+ ions)

* 7: Neutral (equal concentration of H+ and OH- ions)

* 7.1-14: Basic or alkaline (high concentration of OH- ions)

* Hydrogen ions: Acids release hydrogen ions (H+) into solution. The higher the concentration of H+ ions, the lower the pH and the more acidic the solution.

In summary: The concentration of hydrogen ions (H+) is directly related to the pH of a solution. The more H+ ions present, the lower the pH and the more acidic the solution.