Final answer:
An acid-base indicator changes color based on the concentration of hydronium ions in a solution.
Step-by-step explanation:
An acid-base indicator is a substance that displays different colors when in the presence of an acid or a base. The indicator is a weak acid that ionizes within a known pH range. An acid-base indicator changes color based on the concentration of hydronium ions (H3O+) in a solution. In acidic solutions, the hydronium ion concentration is higher, causing the indicator to change to a different color. In basic solutions, the hydronium ion concentration is lower, resulting in another color change.