What is an acid-base indicator?

An acid-base indicator is characterized as a weak acid or base that shows a distinct color change when it undergoes dissociation in solution, which is highly dependent on the pH of the environment. This property arises from the molecular structure of the indicator, which changes its form (and thus its absorption of light) when the pH crosses a specific threshold.

For instance, phenolphthalein is a common acid-base indicator that is colorless in acidic solutions and turns pink in basic solutions. This color change occurs due to the ionization of the indicator itself when the pH changes, thus allowing it to visually signal the acidity or basicity of a solution.

In contrast to this, other options do not accurately describe the nature of an acid-base indicator. While strong acids and bases do exhibit color changes, they are not classified as indicators. Furthermore, the suggestion that an indicator only neutralizes acids or only measures pH does not capture its primary function, which is to visually indicate changes in pH rather than to actively neutralize substances.