Here's why:

* Acids donate protons (H⁺): Acids release hydrogen ions (H⁺) into solution. These hydrogen ions readily combine with water molecules to form hydronium ions (H₃O⁺).

* pH scale: The pH scale measures the acidity or alkalinity of a solution. A lower pH indicates a higher concentration of hydronium ions, making the solution acidic.

* Neutral solutions: In a neutral solution, the concentrations of hydronium and hydroxide ions are equal.

* Basic solutions: Basic solutions, also known as alkaline solutions, have a higher concentration of hydroxide ions than hydronium ions.

In summary: The defining characteristic of an acidic solution is a higher concentration of hydronium ions compared to hydroxide ions.