* Sodium hydroxide is a strong base. This means it completely dissociates in water, releasing hydroxide ions (OH-) which are responsible for the alkaline (basic) nature of the solution.

* Dilution decreases the concentration of hydroxide ions. By adding more water, you spread out the same number of hydroxide ions over a larger volume. This means the concentration of OH- ions decreases.

* Lower hydroxide ion concentration means higher pH. The pH scale is logarithmic, meaning that a decrease in hydroxide ion concentration leads to an increase in pH. A higher pH represents a less alkaline solution.

In summary: Diluting a strong base like sodium hydroxide lowers the concentration of hydroxide ions, resulting in a decrease in pH. The solution becomes less alkaline.