Phenolphthalein is a chemical indicator used to test whether a substance is an acid or an alkali. It is colourless in acidic solutions and turns pink in alkaline solutions, typically in solutions with a pH above 8. This clear colour change makes it useful in experiments to identify pH levels. In GCSE Biology, phenolphthalein may be used in practicals such as investigating enzyme activity or digestion, where pH affects how enzymes function.
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