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Filter paper impregnated with a purple pigment (actually a mixture of several pigments) obtained from dyer’s lichen and other lichen species. As a test paper, it provides a simple means to determine whether a liquid is an acid or a base: when the red test paper is dipped into an alkaline liquid, it turns blue; when blue test paper is dipped into an acid, it turns red. The change of color occurs in a pH range of 5.0-8.0. If no color change takes place, the solution is neutral and has a pH of 7. See also pH-value.