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Mixture of coloured organic compounds obtained from several species of lichens. In the form of a water solution or as litmus paper, it is the oldest and most-used indicator of whether a substance is an acid or a base. It turns red or pink in acid solutions and blue or purple-blue in alkaline solutions.
This entry comes from Encyclopædia Britannica Concise. For the full entry on litmus, visit Britannica.com.