Litmus is a natural dye that can be extracted from certain lichens. It is commonly used as an indicator to determine whether a substance is acidic or alkaline. Litmus paper is a small strip of paper that is coated with litmus dye and is used to test the pH of a solution.
When litmus paper is dipped into an alkaline solution, it turns blue. This is because in an alkaline solution, the concentration of hydroxide ions (OH-) is higher than the concentration of hydrogen ions (H+). The litmus dye reacts with the hydroxide ions in the alkaline solution, causing the paper to turn blue.
On the other hand, when litmus paper is dipped into an acidic solution, it turns red. This is because in an acidic solution, the concentration of hydrogen ions (H+) is higher than the concentration of hydroxide ions (OH-). The litmus dye reacts with the hydrogen ions in the acidic solution, causing the paper to turn red.
Therefore, the colour of litmus in an alkaline medium is blue, while the colour of litmus in an acidic medium is red.