Litmus paper is a type of paper that is used to test the ph level of various substances or solutions. This paper typically comes in strips and is water-soluble. Determining the ph level of a substance basically creates the distinction if it is acidic or basic. To test a solution if it is more acidic than basic, a litmus paper test may be done. In general, solutions that have ph levels of less than 7 are classified as acidic while those are above 7 are classified as basic or alkaline solutions.

Litmus paper is made from a mix of various dyes that are sourced from lichens. These dyes are considered sensitive enough to detect the acidity or alkalinity of solutions. Under normal laboratory conditions, the color of the litmus paper will change depending on the ph level of the solution tested. In the case of blue litmus paper, its color will change to a red hue if it is dipped onto an acidic substance or solution. Red litmus paper meanwhile should turn blue when dipped onto an alkaline solution. Whenever the resulting test will reveal the color purple, it simply means that the solution tested has a neutral ph level. Neutral ph levels pertain to solutions that are neither classified as acidic or alkaline. In most cases, both red and blue litmus paper is used to give more consistent and reliable results. A range of pH values can also be determined through the resulting color change under the litmus paper test. More acidic solutions will have reddish to dark red in color while alkaline solutions will have greenish to dark blue results.

The use of litmus paper in testing the acidity or alkalinity of solutions was said to have started as early as 1300 AD. Its first use is credited to Arnaldus dela Villa Nova, an alchemist that hailed from Spain. Since its first use in chemical laboratories, litmus paper tests continue to become popular in present times despite the fact that there are many more testing procedures that give more specific and accurate ph level results.