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An indicator is a substance which turns different colors in the presence of acids and/or bases. Cabbage water is a common indicator, and is often used in simple acid/base experiments.

Litmus is one name for a lichen, whose extracts turn red in the presence of acids and blue in the presence of bases. Paper soaked in litmus extracts are used in traditional test for detecting acidity or alkalinity. The phrase litmus test has become part of the language, often in a social or political context.

Phenolphthalein is another commonly used indicator, colorless in acid solutions and pink in alkaline solutions. Various other indicator dyes are known, which change color at various pH levels, and by mixing them it is possible to produce universal indicators that turn different, distinct colors at various pH levels. Universal indicator paper can be dipped in a solution, and the approximate pH read off by matching the color against a printed color scale.

In serious chemistry, indicators have largely been displaced by glass-electrode pH meters, which measure the pH electronically to much greater accuracies than indicator dyes.