You may have worked in your chemistry lab with litmus papers as acid-base indicators. They are normally used in experiments that determine the nature of a chemical substance whether acidic or basic. An acidic solution turns blue litmus red and a basic solution turns red litmus blue.
Litmus is a dye obtained from plant-like life forms called lichens. The lichens are slightly different from the ordinary plants in a way that they are actually composed of two plants – a fungus and an alga living in close association for the benefit of each other. The algae are the food producing agents that supply fungus with food. Fungus absorbs moisture that is needed by algae and keeps it anchored to the surface by root like structures.
The lichens are crushed and fermented in a mixture of ammonia and potash. The fermented mixture is blue in color. This is mixed with chalk to form a paste that is dried and powdered. The mixture is absorbed onto a filter paper to turn it into an absorbent litmus paper. Red litmus is made by adding acid.
The commonly used lichen in making litmus papers is rocella tinctoria. There are also few other species from which they can be made. Papers dyed with dye extracted from this variety of lichens are called litmus papers. The dye has large molecules called chromophores. Chromophores are a group of atoms which controls the color of the dye.
The acidic or basic nature of a chemical solution causes a breakage in the bonds of the chromophores altering the range of wavelengths the new structure of chromophores can absorb. That how color changes occur.
Though litmus papers are not completely perfect detectors but are fairly a rough indicator of acids and bases in a fast and easy way.