When a litmus paper is dipped into alkali, it turns blue. Litmus paper is a type of indicator used in chemistry to test the acidity or alkalinity of a solution. It is made from a plant dye that changes colour depending on the pH of the solution. When the pH is below 7, the litmus paper turns red, and when the pH is above 7, it turns blue. Alkali is a substance that has a pH greater than 7, so when litmus paper is dipped into alkali, it turns blue.