What change will you observe if you test soap with litmus paper (red and blue)?

AcademicChemistryNCERTClass 10

Soap is the salt of a strong base (NaOH) and a weak acid (carboxylic acid), so a solution of soap in water is basic in nature. Being basic, a soap solution turns red litmus paper blue.

[Extra information: Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator called Litmus paper. It is a natural indicator.


Litmus solution is extracted from lichens. Lichens are plants belonging to the Thallophyte group.
The process of obtaining a litmus solution needs the crushing and grinding of lichens. These dyes are then added to neutral water to get the required litmus solution. It is then absorbed onto filter paper to produce Litmus paper. 
It is used to test materials for acidity and basicity. This solution can also be used to differentiate between acids and bases. When acid is added to a litmus solution, it changes to red. When a base is added, it changes to blue. Neutral litmus paper is purple in colour.

Updated on 10-Oct-2022 12:47:19