When does litmus paper turn red?

Litmus paper turns red when placed in an acid solution, but blue if the solution is alkaline. This absorbent paper is the oldest and most commonly used indicator of the presence of absence of acid. Its special qualities are due to the fact that it has been soaked and impregnated with a mixture of dyes called litmus.

The litmus mixture was originally produced by the action or air, ammonia and an alkali carbonate on certain lichens found in the Netherlands. It is now made from azolitmin and erythrolitmin. A litmus solution is sometimes used. But the message is the same. A few drops added to a liquid turns it red if acid and blue if alkaline.