Litmus paper is one of the most widely used ways to test the acidity or alkalinity of a solution. It is also one of the oldest ways to test for those properties, having been used for that purpose of several centuries. It has the added virtues of being inexpensive and easy to buy.
Acid or Base?
Chemical substances are generally either acid or alkaline (also called base). Acids release hydrogen ions -- positively charged hydrogen atoms -- in the presence of water, while alkaline (base) substances release hydroxide ions, or negatively charged OH molecules.
The pH Scale
The measurement of acidity and alkalinity is called the pH scale, numbering from 0 to 14. A number less than 7 indicates acid, with the smaller number being more strongly acidic; greater than 7 indicates alkaline (base), with the greater number being more alkaline (basic).
Litmus paper is useful to test, quickly and reliably, whether a solution is acid or alkaline (base) -- that is, greater or less than 7 on the pH scale. It changes colour depending on what it is exposed to. It cannot, however, determine an exact pH number of a solution.
Litmus Paper Composition
Litmus paper is paper treated with various extracts of lichen, a kind of fungus that also has characteristics of algae, to make different kinds of paper: red or blue. It is also possible to use synthetic materials to mimic the effects of the lichen extract.
Red or Blue Litmus Paper
Red litmus paper turns blue in the presence of alkaline (base) solution, while blue litmus paper turns red when exposed to an acidic solution. In the presence of a neutral solution (pH 7), such as distilled water, litmus paper does not change colour.
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