Here's why:

* Acids release hydrogen ions (H+) in solution, making the solution more acidic.

* Bases accept hydrogen ions (H+) in solution, making the solution more basic (also called alkaline).

This relationship is described by the pH scale, which measures the acidity or basicity of a solution:

* pH 0-6.9: Acidic

* pH 7: Neutral

* pH 7.1-14: Basic (alkaline)

So, acids and bases are opposites in terms of their chemical properties and how they interact with hydrogen ions.