What is a substance used as an acid-base indicator?
One substance used as an acid-base indicator is phenolphthalein.
Phenolphthalein is clear in acidic solutions, but turns bright pink in basic solutions. This indicator is used when titrating an acidic solution with a basic solution of known concentration. When the endpoint of the titration is reached, the solution will change from clear to pink.
This video shows an example of a titration experiment.
There are other substances which will display a wider range of colors depending on their pH level.
The video below shows an experiment using an indicator derived from boiling red cabbage. A pigment from the cabbage called anthocyanin is what causes all of the different colors you see.
Other common indicators include:
Hope this helps!