Question

Name a gas which turns moist blue litmus red and then bleaches it.

Answer 1

The analysis which aims at detecting the presence of any substance in a given sample is called as qualitative analysis.

• On reacting chlorine (Cl₂) gas with water from the moist litmus paper, chlorine gas forms Hydrochloric acid (HCl) which is responsible for the release of H⁺ ions. Due to this, chlorine gas behaves as an acid and turns blue litmus red.

$$\begin{gathered} {\mathrm{Cl}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{HCl}\left(\mathrm{aq}\right)+\mathrm{HOCl}\left(\mathrm{aq}\right) \\ \left(\mathrm{Chlorine}\right)\left(\mathrm{Water}\right)(\mathrm{Hydrochloric}\mathrm{acid})(\mathrm{Hypochlorous}\mathrm{acid}) \end{gathered}$$

• Further, chlorine gas also acts as a bleaching agent. It reacts with water to form Hypochlorous acid (HOCl) which acts as an oxidising agent and gives off an oxygen atom. This oxygen atom reacts with dye and makes it colourless. Thus, the bleaching action of Chlorine gas is by oxidation.

$$\begin{gathered} \mathrm{HOCl}\left(\mathrm{aq}\right)+\mathrm{Dye}\to \mathrm{HCl}\left(\mathrm{aq}\right)+[\mathrm{Dye}-\mathrm{O}] \\ (\mathrm{Hypochlorous}(\mathrm{responsible}\mathrm{for}\mathrm{colour})(\mathrm{Hydrochloric}(\mathrm{responsible}\mathrm{for} \\ \mathrm{acid})\mathrm{acid})\mathrm{no}\mathrm{colour}) \end{gathered}$$

Thus, Chlorine (Cl₂) gas turns moist blue litmus red and then bleaches it.