What is litmus?

Litmus is a mixture of dyes obtained from litchens. It is
mainly used to test the acidity or alkalinity of a solution. There are red and blue
litmus papers. If a red litmus paper is put into an alkaline solution, it will turn blue
and if a blue litmus paper is put into an alkaline solution, it will remain blue (no
change).

If a blue litmus paper is put into an acidic
solution, it will turn red and If a red litmus paper is put into an acidic solution,
there will be no change.