MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Soil sample being tested for pH using universal indicator. Universal indicator (UI) is a mixture of chemicals that changes colour depending on the pH (acidity) of its environment. This solution has turned green, showing that it has a pH near 7. The pH scale runs from 1 (strong acid) through 7 (neutral) to 14 (strong alkali). In strong acids it is red, turning orange then yellow as the pH increases. In water it is green, and it turns blue then purple in successively stronger alkalis.
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