MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Universal indicator in a weak acid. Universal indicator is a mixture of chemicals that changes colour according to the pH (acidity) of the solution in which it is placed. It is red in strong acids, orange in weak acids, yellow/green in neutral water, and blue then purple in increasingly strong alkalis.
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