Litmus paper is made from filter paper treated with dyes extracted from lichens that change color in response to pH changes. It tests for acidic or basic substances, but not their exact pH. The pH scale measures acidity or alkalinity from 0-14, with 7 being neutral. Universal indicator-treated pH paper shows where a sample falls on the scale.
A litmus test uses litmus paper to determine whether a substance is alkaline or acidic. Litmus paper is made from a strip of filter paper that has been treated with litmus, a mixture of dyes extracted from lichens. These dyes are pH indicators or substances that change color in response to a change in pH. A litmus paper uses weak acids and bases to test for stronger acids and bases. In a typical litmus test, a red stripe of litmus paper will turn blue if the substance being tested is basic and a blue stripe will turn red if the substance is acidic. No color change indicates that the substance is neutral.
pH is a measure of the concentration of hydroxide ions (OH-) and protons called hydrogen ions (H+) in a solution. When added to water, acids are proton donors because they cause an increase in dissolved hydrogen ions. Bases can be thought of as proton acceptors because they bind to hydrogen ions, causing less H+ to be lost in the solution. Lemon juice, for example, is acidic because it adds H+ into solution, and ammonia is basic, or alkaline, because it absorbs hydrogen ions in the mixture. Both bases and acids react readily with other substances, which is why stomach acid is made to break down food and bases are used to cut dirt in cleaning products.
The litmus powder that is used to treat pH strips is extracted from lichens, an organism that thrives in a wide variety of climates, including the extreme environment of the tundra. Lichens are an organism composed of symbiotic relationships between fungi and algae. Of the many types of lichens that exist, Rocella tinctoria, Dendrographa leucophoea, and Decanora tartarea are the varieties most commonly infused into litmus paper. Litmus strips are naturally blue, and red strips are created by further treating the strip with a weak acid.
A litmus paper only shows whether a substance is acidic or basic, not where it falls on the pH scale. The pH scale measures the degree of acidity or alkalinity from zero to fourteen, with zero being the strongest acid, fourteen being the strongest base, and seven being neutral. To find out how acidic or basic a sample is, you need to use a pH paper. pH paper is treated with a compound called a universal indicator that changes color to show where a sample lands on the pH scale. If a sample causes the test strip to turn red, the sample is between zero and three on the pH scale. A yellow stripe indicates a sample between three and six, a green stripe indicates a neutral sample, a blue stripe indicates a base between eight and eleven, and a purple stripe indicates a strong base.
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