Question

Name two coloured gases.

Answer 1

Two coloured gases are Nitrogen dioxide (NO₂) and Chlorine (Cl₂) gas.

1. Nitrogen dioxide (NO₂) is reddish brown in color.
2. Chlorine (Cl₂) is greenish yellow in colour.

These distinct colours help in the qualitative analysis of these gases.

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

Evolution of Nitrogen dioxide takes place as follows: On heating crystals of Lead nitrate [(Pb(NO₃)₂], it produces a crackling sound first. On further heating, dense reddish brown fumes of Nitrogen dioxide (NO₂) are seen.

$$\begin{gathered} 2\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)\stackrel{∆}{\to }2\mathrm{PbO}\left(\mathrm{s}\right)+4{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ (\mathrm{Lead}\mathrm{nitrate})(\mathrm{Lead}\mathrm{oxide})(\mathrm{Nitrogen}\mathrm{dioxide})\left(\mathrm{Oxygen}\right) \\ (\mathrm{Reddish}\mathrm{brown}) \end{gathered}$$

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).

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