Litmus is used in chemistry to detect the presence of alkalis and acids. Litmus is a dye made from small plants called lichens. It is either red or blue in colour and is used in the form of a solution which is sometimes on a test paper.
When lichen called Rocella Tincotoria is allowed to react with ammonia, potassium carbonate or lime, it gives a blue colour material. The paper is dipped into it and dried. This is known as a blue litmus paper, and is used to test acids. Acids turn blue litmus red.
Orchil or cudbear is a red dye obtained from another species of lichens. This is used to make red litmus paper. Alkannet or alkanna is another dye obtained from the root of the plant Alkanna Tinotoria. The colouring ingredient, alkannin, is soluble in alcohol, benzene and others. When white paper is impregnated with an alcoholic solution of alkannet, it becomes red. This red paper is turned blue or deep violet by alkalis.
Neutral solutions (neither acid nor base) do not change the colour of litmus.
When a chemist wishes to neutralize an acid solution, he first adds litmus solution. This changes its colour to red. The base is then added, until its colour changes to violet. The solution then becomes neutral, i.e. neither base nor acid and one more drop of base turns the solution blue.
When acids and bases react, they produce salts by neutralization. For instance, the common salt that we use in our food is produced by the reaction of caustic soda and hydrochloric acid.
Nowadays, litmus paper is made from several substances such as azolitmin, crysthrolitmin, spaniolitmin etc. These are apparently mixtures of closely-related compounds that were identified in 1961 as derivatives of the heterocyclic compound phenoxazine.