Question

What is a substance used as an acid-base indicator?

Answer 1

One substance used as an acid-base indicator is phenolphthalein.

Explanation:

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:
bromothymol blue
thymol blue
methyl orange
bromocresol green
methyl red
phenol red

Hope this helps!