Chemistry: acid will turn litmus paper what color?

Litmus paper serves as a classic indicator in chemistry, revealing whether a substance is acidic or basic. Its ability to change color stems from a natural, water-soluble dye extracted primarily from lichens. This dye is sensitive to the concentration of hydrogen ions (H+) in a solution, which is the fundamental principle behind the pH scale. When a solution contains a high concentration of these hydrogen ions, it is considered acidic, and the dye within the litmus paper reacts to this presence.

Specifically, when blue litmus paper encounters an acidic substance, the dye undergoes a chemical transformation, causing the paper to turn red. Conversely, if red litmus paper is dipped into an acidic solution, it will not change color, remaining red. The change to blue only occurs when red litmus paper is exposed to an alkaline or basic solution. Therefore, the unmistakable shift from blue to red is the key visual cue indicating that the tested liquid has an acidic nature, typically corresponding to a pH value below 7.

The history of litmus paper is quite rich, with its initial documented use dating back to around 1300 CE by Spanish physician Arnaldus de Villa Nova. The unique color-changing properties of the lichen-derived dye have made it an invaluable tool for centuries. Beyond its scientific applications in laboratories and educational settings, the phrase "litmus test" has even permeated everyday language, referring to any critical assessment that reveals the true character of something, highlighting its enduring significance.