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In politics, a “litmus test” is a metaphor for a decisive question that reveals a candidate’s core beliefs. In chemistry, a litmus test is a practical method for determining whether a liquid is acidic or alkaline. It involves dipping strips of specially treated paper into the solution and observing a color change.
Litmus paper comes in two primary varieties: blue and red. Each type reacts differently to acids and bases, enabling a quick assessment of a solution’s nature.
Blue litmus paper turns red when it contacts an acidic solution with a pH below 4.5, while it remains blue in the presence of a base. Conversely, red litmus paper turns blue when it encounters a basic solution with a pH above 8.3, and stays red when exposed to acids. In practice, you can use either paper to test a substance, remembering that red indicates acidity and blue indicates basicity.
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Both blue and red litmus papers are produced by treating ordinary paper with a blend of 10 to 15 dyes derived from lichens—a symbiotic organism composed of fungus and algae. The most commonly used species is Rocella tinctoria, one of over 3,000 lichen species found in North America.
The color change is governed by the chromophore 7‑hydroxyphenoxazone. In acidic environments, it gains a proton and reflects red light; in basic environments, it loses a proton and reflects blue light. Importantly, all litmus paper is naturally blue; red litmus paper is simply blue paper that has been acid‑treated to display red, reverting to blue when it contacts a base.
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While litmus paper offers a rapid binary indication of acidity or alkalinity, it does not provide a precise pH value. Red litmus paper signals acids only when the pH is below 4.5, and blue litmus paper indicates bases only when the pH exceeds 8.3. The full pH scale ranges from 0 to 14, with 7 being neutral.
Universal indicator strips, in contrast, incorporate synthetic dyes such as thymol blue, methyl red, bromothymol blue, and phenolphthalein. These strips begin yellowish and shift through a continuous spectrum—yellow to red for increasing acidity and blue to purple for increasing alkalinity—often marking neutral pH with green. This broader color range allows for a more accurate estimation of the exact pH.
