Question

Identify the gas from the following observations:

A greenish-yellow gas having a pungent and suffocating smell. It turns moist blue litmus paper red and then bleaches it.

Answer 1

Chlorine gas (Cl₂) is greenish-yellow in colour. This colour helps in its qualitative analysis. It has a sharp choking odour or suffocating smell.

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

The evolution of Chlorine gas takes place as follows: Manganese dioxide (MnO₂) when heated with concentrated Hydrochloric acid (HCl) to produce Chlorine gas (Cl₂) along with Manganese chloride (MnCl₂) and water (H₂O).

$$\begin{gathered} {\mathrm{MnO}}_2\left(\mathrm{s}\right)+4\mathrm{HCl}\left(\mathrm{l}\right)\stackrel{∆}{\to }{\mathrm{MnCl}}_2\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right) \\ (\mathrm{Manganese}\mathrm{dioxide})(\mathrm{Hydrochloric}\mathrm{acid})(\mathrm{Manganese}\mathrm{chloride})\left(\mathrm{Water}\right)\left(\mathrm{Chlorine}\right) \end{gathered}$$

The chlorine gas liberated here is greenish-yellow in colour.

• 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 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.