TREVOR CLIFFORD PHOTOGRAPHY / SCIENCE PHOTO LIBRARY TREVOR CLIFFORD PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Adding acid to an alkali. Beakers of sodium hydroxide solution (NaOH, top), and hydrochloric acid solution (HCl, left), and bottle of universal indicator (right). The beaker in the middle contains NaOH and universal indicator mixed together. HCl has been added with a pipette. Universal indicator is used to indicate acidity or alkalinity. In the bottle it is green (neutral). In the middle beaker it is also green as adding the acid has neutralised the alkaline sodium hydroxide.
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