The quantity that tells you whether a solution is acidic or basic is the pH.

Here's how it works:

* pH scale: The pH scale ranges from 0 to 14.

* Acidic solutions: Have a pH less than 7. The lower the pH, the stronger the acid.

* Neutral solutions: Have a pH of 7. Pure water is neutral.

* Basic (alkaline) solutions: Have a pH greater than 7. The higher the pH, the stronger the base.

Important note: pH measures the concentration of hydrogen ions (H+) in a solution. Acids release H+ ions when dissolved, while bases accept H+ ions.