I think you are referring to litmus in the context of the litmus test which is performed to identify whether a substance is acidic or alkaline. Litmus is a mixture of different dyes. These are extracted from different types of lichen. The dye is usually used after it has been applied on paper and can then be used to measure pH in the range 4.5- 8.3. Litmus paper turns red when it is exposed to acidic substances and blue when it is exposed to alkaline substances. The use of litmus in the study of chemicals is quite old with the first use dating back to 1300 AD.
Litmus is a mixture of dyes obtained from litchens. It is mainly used to test the acidity or alkalinity of a solution. There are red and blue litmus papers. If a red litmus paper is put into an alkaline solution, it will turn blue and if a blue litmus paper is put into an alkaline solution, it will remain blue (no change).
If a blue litmus paper is put into an acidic solution, it will turn red and If a red litmus paper is put into an acidic solution, there will be no change.
Litmus is a water-soluble mixture of different dyes extracted from lichens, specially Roccella tinctoriaThe mixture has CAS number 1393-92-6. It is often absorbed on to filter paper. The resulting piece of paper or solution with water becomes a pH indicator (one of the oldest), used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (i.e. alkaline) conditions, the color change occurring over the pH range 4.5-8.3 (at 25°C). The color changes occur at about 8.3 and 4.5 pH units respectively.Neutral litmus paper is purple in color.[