What is the function of an acid-base indicator?

An acid-base indicator serves the specific function of changing color in response to variations in pH. This characteristic allows the indicator to visually signal the acidity or alkalinity of a solution. When an acid-base indicator is added to a solution, it undergoes a chemical change that results in a color shift depending on the pH level of that solution.

This property is crucial in various titrations and pH testing, allowing chemists to determine the point of neutralization or to approximate the pH of a solution without needing sophisticated equipment. For example, phenolphthalein changes from colorless in acidic solutions to pink in basic solutions, providing immediate visual feedback about the solution's pH state.

The other options do not accurately describe the primary function of an acid-base indicator, as they relate to different concepts within chemistry.