Olfactory indicators are substances whose odor changes in acidic or basic media. Unlike acid-base indicators, which change color, olfactory indicators rely on changes in smell to indicate the presence of acids or bases.

Acid-base indicators are commonly used to detect the presence of acids or bases in a solution by observing changes in color. They are valuable tools in laboratories and everyday applications for determining the pH of substances.

Besides litmus solution, name three other natural materials used as acid-base indicators.

When litmus solution is neither acidic nor basic, its color is purple.

Litmus solution is a purple dye extracted from lichen, a plant belonging to the division Thallophyta.