Here's why:

* Dilution means adding more solvent (usually water) to a solution.

* Concentration refers to the amount of solute (in this case, the alkali) present in a given amount of solution.

When you dilute an alkali, you are decreasing the concentration of the alkali. You are spreading the same amount of alkali over a larger volume of solution, making the alkali less concentrated.

Think of it like this: Imagine you have a cup of strong coffee. If you add water to it, you are making the coffee less concentrated, or weaker.