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Indicator Definition. A chemical indicator is a substance that undergoes a distinct observable change when conditions in its solution change. This could be a color change, precipitate formation, bubble formation, temperature change, or other measurable quality.
indicator organisms. Species very sensitive to changes in the environment, and whose presence or absence may be indicative of environmental conditions in a particular area or habitat. They serve the same purpose as the caged birds kept in old mines whose sudden death indicated presence of carbon monoxide.