GUSTOIMAGES / SCIENCE PHOTO LIBRARY GUSTOIMAGES / SCIENCE PHOTO LIBRARY
Universal indicator paper. This paper is used to measure the strength of an acid or an alkali. When the paper is dipped into a solution, it changes colour depending on the solution's pH. The purple and dark blue papers have been dipped in alkaline or basic solutions (approximately pH 11, 10 and 9 from left to right). The green papers are slightly alkali (pH 8, centre left), neutral (pH 7, centre), and slightly acid (pH 6, centre right). The orange and red papers have been dipped in acidic solutions (approximately pH 4, 3 and 2 from left to right).
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