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Is the glass half empty or half full?, a litmus test for somebody's worldview, photo © JWG
Is the glass half empty or half full?, a litmus test for somebody's worldview, photo © JWG

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Kunstformen der Natur (1904) by Ernst Haeckel
Kunstformen der Natur (1904) by Ernst Haeckel

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 with the color change occurring over the pH range 4.5-8.3 at 25 °C. Neutral litmus paper is purple in color. Litmus can also be prepared as an aqueous solution that functions similarly. Under acidic conditions the solution is red, and under basic conditions the solution is blue.

Litmus test

The main use of litmus is to test whether a solution is acidic or basic. Wet litmus paper can also be used to test water-soluble gases; the gas dissolves in the water and the resulting solution colors the litmus paper. For instance, ammonia gas, which is alkaline, colors the red litmus paper blue.

Chemical reactions other than acid-base reaction can also cause a color-change to litmus paper. For instance, chlorine gas turns blue litmus paper white – the litmus dye is bleached, due to presence of hypochlorite ions. This reaction is irreversible and therefore the litmus is not acting as an indicator in this situation.

Unless indicated otherwise, the text in this article is either based on Wikipedia article "Litmus" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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