Here's why:

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

* Bases release hydroxide ions (OH-) in solution.

Therefore, a solution with a higher concentration of hydronium ions than hydroxide ions indicates that more acid has been added to the solution, making it acidic.

Examples of acidic solutions include:

* Lemon juice

* Vinegar

* Stomach acid

Key takeaway: The relative concentrations of hydronium and hydroxide ions determine the acidity or alkalinity of a solution.