right|thumb|Litmus powder right|thumb|Chemical structure of 7-hydroxyphenoxazone, the chromophore of litmus components
Litmus is a natural dye extracted from certain lichens that changes color depending on whether a substance is acidic or basic, making it useful for simple chemical testing. It has been historically important in chemistry and remains relevant today as a practical tool for measuring acidity levels in laboratories and educational settings.
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right|thumb|Litmus powder right|thumb|Chemical structure of 7-hydroxyphenoxazone, the chromophore of litmus components
Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. In an acidic medium, blue litmus paper turns red, while in a basic or alkaline medium, red litmus paper turns blue. In short, it is a dye and indicator which is used to place substances on a pH scale.
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