What are acid-base indicators? Give one example of synthetic indicator and olfactory indicator.

Hint: The acid-base indicators use the Arrhenius theory of acid base. The acidic substances are detected and identified on the basis of their ability to release hydrogen ions and the bases are identified on the basis of their ability to release hydroxide ions.

Complete answer:
The acid-base indicators are naturally occurring organic substances or chemically synthesized dyes that are sensitive to a change in pH and show visible physical changes that help in distinguishing between acids and bases.
The Arrhenius theory of acids and bases states that the substances capable of releasing hydrogen ions are acidic and the substances capable of releasing hydroxide ions are basic in nature.
The indicators show a change in their chemical structure on accepting the hydrogen or hydroxide ions released by acids or bases respectively which brings a change in their colour or odour.
Chemical dyes that are synthesized to serve as indicators are known as synthetic indicators. Examples include: methyl orange, phenolphthalein and Thymol blue.
Indicators that help determine a change in pH by a distinctive odour are known as olfactory indicators. Examples include onions, vanilla extract and cloves.

Note:
The indicators used for detecting acids and bases are chosen according to the strength of the acid or base that needs to be detected. All indicators do not show visible changes in colour or odour at all ranges of pH. Different indicators are effective within a small range of pH only.