Damp litmus paper undergoes a color change when exposed to chlorine due to chlorine’s strong oxidizing properties, which bleach the litmus paper.

Chlorine, a highly reactive element, is categorized as a halogen in the periodic table. It has a strong inclination to gain electrons, making it an effective oxidizing agent. When damp litmus paper comes into contact with chlorine gas, the chlorine molecules react with the water molecules present in the paper, resulting in the formation of hydrochloric acid and hypochlorous acid. The chemical reaction can be represented as:

The produced hypochlorous acid ($\text{HClO}$) further interacts with the dye in the litmus paper, causing it to lose its color. This phenomenon is referred to as bleaching. The bleaching effect of chlorine is so potent that it can render the litmus paper white, regardless of whether the initial color was blue or red.

It is essential to recognize that the color change in litmus paper is typically used to detect acidity (where blue litmus paper turns red) or alkalinity (where red litmus paper turns blue). However, in the case of chlorine, the observed color change does not indicate acidity or alkalinity; instead, it showcases the bleaching effect stemming from chlorine’s strong oxidizing capabilities.

In summary, the color change of damp litmus paper in the presence of chlorine is a direct result of the chemical reactions facilitated by chlorine’s powerful oxidizing nature. This behavior is a fundamental characteristic of chlorine and other halogens, and it plays a crucial role in understanding their reactivity and interactions in various chemical processes.