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Universal indicator test on a lemon. A piece of universal indicator paper held against the flesh of a freshly cut lemon. The change of colour from yellow to red shows that the lemon is very strongly acidic. Lemons, like all citrus fruits, contain citric acid. Universal indicator paper changes its colour depending on the pH of the solution tested. It is used to measure the strength of an acid or an alkali. Acid solutions have a high concentration of hydrogen ions, while alkali solutions have a low concentration of hydrogen ions.
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