MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Litmus pH indicator in acid (left) and base (right). Litmus is a water soluble mixture of dyes extracted from lichens. When used as a test, acids below pH 4.5 turn the dye red and bases above pH 8.3 turn the dye blue.
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