* pH Scale: The pH scale measures the acidity or alkalinity of a solution. A lower pH indicates higher acidity, while a higher pH indicates higher alkalinity.

* Dilution: When you add water to a base, you are diluting it. This means you are decreasing the concentration of hydroxide ions (OH-), which are responsible for the base's alkalinity.

* Equilibrium: The concentration of H+ and OH- ions in water is governed by an equilibrium. When you dilute a base, the equilibrium shifts to favor the production of more H+ ions, resulting in a lower pH.

In simpler terms: Adding water to a base makes it less "basic" or less alkaline, which means the pH will decrease.