Why do acids change blue litmus to red?
Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, 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 pigment in blue litmus reacts with
Otherwise the same pigment in base reacts differently with O