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In Chemistry, What Is the Litmus Test?

Litmus tests determine whether a substance is acidic or alkaline.
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  • Written By: Caitlin Kenney
  • Edited By: Bronwyn Harris
  • Last Modified Date: 30 August 2014
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A litmus test uses litmus paper to determine whether a substance is alkaline or acidic. Litmus paper consists of 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 test uses weak acids and bases to test for stronger acids and bases. In a typical litmus test, a red strip of litmus paper will turn blue if the tested substance is basic and a blue strip will turn red if the substance is acidic. No color change indicates that the substance is neutral.

pH is a measurement of the concentration of hydroxide ions (OH-) and protons called hydrogen ions (H+) in a solution. When added to water, acids are proton donators because they cause an increase in loose hydrogen ions. Bases can be thought of as proton acceptors because they bind with hydrogen ions, causing less H+ to be loose in the solution. Lemon juice, for example, is acidic because it adds H+ in solution, and ammonia is basic, or alkaline, because it takes up the hydrogen ions in the mixture. Both bases and acids react readily with other substances, which is why stomach acid is produced to break down food and bases are used to cut grime in cleaning products.

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The litmus powder that is used to treat the 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 in litmus test paper. The litmus strips are naturally blue and red strips are creating by further treating the strip with a weak acid.

A litmus test 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 how acidic or basic a sample is, a pH paper must be used. pH paper is treated with a compound called a universal indicator which changes colors to show where a sample lands on the pH scale. If a sample turns the test strip red, the sample is between zero and three on the pH scale. A yellow strip indicates a sample between three and six, a green strip indicates a neutral sample, a blue strip indicates a base between eight and eleven, and a purple strip indicates a strong base.

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