Litmus is a natural indicator .
- It is a purple coloured dye which is extracted from a plant called Lichen .
- It is also available in paper form.
- When chemicals are added to it, it changes colour according to the nature of the substance .
- When the chemical is neutral , its colour remains unchanged (purple).
- When the chemical is acidic in nature, its colour changes to red.
- When the chemical is basic in nature, its colour changes to blue.