Introduction:
Litmus paper is a commonly used indicator in chemistry to determine whether a substance is an acid or a base. It is made from a water-soluble mixture of different dyes extracted from lichens. The paper changes color when it comes into contact with an acidic or basic solution.

Dry Litmus Paper:
When litmus paper is dry, it is typically red in color. This is because the paper contains a dye called azolitmin, which is red in acidic solutions but turns blue in basic solutions. The color change occurs due to the reaction between the dye and the hydrogen ions (H+) or hydroxide ions (OH-) present in the solution.

Wet Litmus Paper:
When litmus paper is wet, it becomes more sensitive to changes in pH. This is because the water on the paper allows for better contact between the dye and the solution being tested. When wet litmus paper is exposed to an acidic solution, it turns red. On the other hand, when it comes into contact with a basic solution, it turns blue.

Color Change Explanation:
The color change in litmus paper occurs due to a chemical reaction between the dye and the ions present in the solution. In an acidic solution, litmus paper turns red because the azolitmin dye reacts with the hydrogen ions (H+) present in the solution. This reaction causes a shift in the electron distribution within the dye molecule, leading to a change in color.

In a basic solution, litmus paper turns blue because the azolitmin dye reacts with the hydroxide ions (OH-) present in the solution. This reaction also causes a shift in the electron distribution within the dye molecule, resulting in a change in color.

Summary:
In summary, when dry litmus paper is exposed to an acidic solution, it turns red, while in a basic solution, it remains red. However, when wet litmus paper is exposed to an acidic solution, it turns red, and in a basic solution, it turns blue. The color change occurs due to a chemical reaction between the azolitmin dye in the litmus paper and the hydrogen or hydroxide ions present in the solution.