Define an indicator and discuss the criteria for the selection of an acid-base indicator with the help of an example.  

An indicator is a substance that undergoes a distinct color change or some other observable change in physical properties in response to changes in pH. In acid-base titrations or any other pH-dependent reactions, indicators are used to determine the endpoint, which is the point at which the stoichiometrically equivalent amounts of acid and base have reacted. Indicators provide a visual indication of the pH of the solution being tested.

Criteria for the selection of an acid-base indicator:

1. pH Range: The indicator should have a pH range that is suitable for the particular acid-base titration or pH measurement being performed. It should undergo a color change within the pH range of the expected endpoint of the reaction. For example, if the expected endpoint is around pH 7, an indicator that changes color within the range of pH 6-8 would be appropriate.

2. Color Change: The indicator should exhibit a sharp and easily distinguishable color change at the endpoint. _____ ________ _____ ____ _____.
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