pH paper, also called litmus paper, is filter paper that is treated with natural water soluble dye from lichens. pH paper is used as an indicator to test the acidity of water-based liquids.
Litmus paper is often used to differentiate an acid from a base. When pH is less than 4.5, the paper turns red. This indicates that the substance is an acid. If pH is greater than 8.3, the paper turns blue, indicating that the substance is a base. In neutral conditions, pH paper is a purple color. The exact pH of a substance can be determined with a pH meter.