What is litmus paper used for? To indicate if all the reactants have been used up to determine if a chemical reaction has occurred to determine pH to detect the presence of water

The main use of litmus is to test whether a solution is acidic or basic. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the pH range 4.5-8.3 at 25 °C (77 °F).