MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Acidity indicator colours. Three test tubes containing an indicator solution and, from left to right, an acid, neutral water, and an alkali. An indicator is a chemical or mixture of chemicals that has a different colour depending on the pH (acidity) of the solution it is added to.
Model release not required. Property release not required.