GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY
Universal indicator added to two test tubes containing different solutions. Left: diluted hydrochloric acid (acidic). Right: diluted sodium hydroxide (alkaline). Universal indicator changes its colour depending on the pH of the solution. Red indicates a strong acid, yellow/orange indicates an acid, green is neutral, blue indicates an alkali, and purple indicates a strong alkali.
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