ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Acid and alkali indicators are chemicals that change colour when mixed with acid or alkali. The phenolphthalein (left dropper bottle) and methyl orange (right dropper bottle) have been added to equal volumes of acidic and alkaline solutions. The phenolphthalein remains colourless in acid but turns pink in alkali. The methyl orange indicator turns red in acid and yellow in alkali.
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