Here's why:

* Acidity and alkalinity (or basicity) are terms used to describe the pH scale, which measures the concentration of hydrogen ions (H+) in a solution.

* Acidic solutions have a high concentration of H+ ions and a pH less than 7.

* Alkaline or basic solutions have a low concentration of H+ ions and a pH greater than 7.

So, if something is acidic, the opposite would be alkaline or basic.