ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Universal indictor in alkali. The colour of the solution is being compared to a chart on the bottle of indicator, showing that it has pH 14. Universal indicator (UI) is a mixture of chemicals that changes colour depending on the pH (acidity) of its environment. The pH scale runs from 1 (strong acid) through 7 (neutral, such as water) to 14 (strong alkali). In strong acids it is red, turning orange then yellow as the pH increases. In water it is yellow/green, and it turns blue then purple in successively stronger alkalis. See images A500/487 and A500/488 for a acidic and neutral solution respectively.
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