Litmus, a well-known acid-base indicator, is derived from :

Litmus is a widely used natural acid-base indicator that is extracted from lichens. Lichens are unique composite organisms consisting of a symbiotic association between a fungus and an alga (or cyanobacterium).

• Source: The dye is primarily obtained from various species of lichens, such as Roccella tinctoria and Lecanora tartarea.
• Function: It is used to determine the acidity or alkalinity of a substance. In its neutral state (distilled water), litmus has a mauve (purple) color.
• Chemical Reaction: When exposed to an acidic solution, litmus turns red. Conversely, when exposed to a basic (alkaline) solution, it turns blue.

As per NCERT Science standards, litmus is the most commonly used natural indicator, typically available either as a solution or in the form of paper strips (litmus paper).