Here's why:

* pH Scale: The pH scale measures the acidity or alkalinity of a solution. A lower pH indicates a more acidic solution, while a higher pH indicates a more basic (alkaline) solution.

* Neutralization: When an acidic solution is added to a basic solution, a neutralization reaction occurs. This reaction involves the hydrogen ions (H+) from the acid reacting with the hydroxide ions (OH-) from the base to form water (H2O).

* pH Change: As the hydrogen ions (H+) from the acid are consumed in the neutralization reaction, the concentration of H+ in the solution decreases. This decrease in H+ concentration leads to a decrease in the acidity of the solution, resulting in a lower pH value.

In simpler terms, adding acid to a base makes the mixture less basic (and therefore more acidic), which is reflected in a lower pH.