Litmus test (chemistry)

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Litmus (pH indicator)
below pH 4.5		above pH 8.3
4.5	↔	8.3

Litmus is a water-soluble dye extracted from certain lichens and absorbed on to filter paper. The resulting piece of paper 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). Neutral litmus paper is purple in colour. The active ingredient of Litmus is called Erythrolitmin, and commercial litmus dye is chemically p-Dimethylaminoazobenzenesulfonic acid, sodium salt. Look up NASA for more details.

To test the pH of some gases using litmus paper, wet litmus paper is needed. The gas dissolves in the water and thus the reaction can be seen.

The most common litmus reactions are when red litmus turns blue or blue litmus turns red. However, these aren't the only reactions. For example, chlorine gas turns blue litmus paper white—chemically speaking, the litmus paper is bleached.