Olfactory indicators are substances that have a distinct odor and change their smell when added to acidic or basic solutions. They can be used in the laboratory to test whether a solution is acidic or basic. When an olfactory indicator is added to a solution, the change in odor can indicate whether the solution is acidic or basic. For example, onion and vanilla extracts are two olfactory indicators.

The working principle of an olfactory indicator is based on the fact that when an acid or base is added to it, different odors can be detected in bases, whereas the odor remains the same in acids. The properties of olfactory indicators include having a distinct odor in acidic and basic solutions. When sodium hydroxide solution is added to an olfactory solution like onion juice, the smell of onion cannot be detected.