Why does damp litmus paper change colour in chlorine?

Damp litmus paper changes colour in chlorine because chlorine is a strong oxidising agent that bleaches the litmus paper.

Chlorine is a highly reactive element that belongs to the group of halogens in the periodic table. It has a strong tendency to gain electrons, making it a potent oxidising agent. When damp litmus paper is exposed to chlorine gas, the chlorine molecules react with the water molecules present in the damp litmus paper to form hydrochloric acid and hypochlorous acid. This reaction is as follows:

Cl2(g) + H2O(l) → HCl(aq) + HClO(aq)

The hypochlorous acid (HClO) then further reacts with the litmus dye, causing it to lose its colour. This process is known as bleaching. The bleaching effect of chlorine is so strong that it can turn the litmus paper white, regardless of whether the paper was initially blue or red.

It's important to note that the colour change in litmus paper is typically used to test for acids (turning blue litmus paper red) or alkalis (turning red litmus paper blue). However, in the case of chlorine, the colour change is not indicating acidity or alkalinity, but rather the bleaching effect of chlorine's strong oxidising properties.

In conclusion, the colour change of damp litmus paper in chlorine is a result of the chemical reactions that occur due to chlorine's strong oxidising nature. This is a key characteristic of chlorine and other halogens, and is a fundamental concept in understanding their reactivity and behaviour in various chemical reactions.