Question

Identify the gas from the following observations:

A colourless gas having a suffocating smell. It turns limewater milky and potassium dichromate solution green.

Answer 1

Sulphur dioxide (SO₂) is a colourless gas which turns lime water milky and has suffocating smell.

• Limewater is an aqueous solution of Calcium hydroxide [Ca(OH)₂].

Sulphur dioxide reacts with limewater to produce a white precipitate of Calcium sulphite (CaSO₃) which makes the solution look cloudy or milky in appearance.

• $$\begin{gathered} {\mathrm{SO}}_2\left(\mathrm{g}\right)+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)\to {\mathrm{CaSO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Sulphur}\mathrm{dioxide})(\mathrm{Calcium}\mathrm{hydroxide})(\mathrm{Calcium}\mathrm{sulphite})\left(\mathrm{Water}\right) \end{gathered}$$

Sulphur dioxide (SO₂) reacts with Potassium dichromate (K₂Cr₂O₇) solution and changes its colour from orange to green.

• It is a redox reaction where oxidation and reduction take place simultaneously.
• Potassium dichromate acts as an oxidising agent and Sulphur dioxide gas acts as a reducing agent.
• $$\begin{gathered} 3{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{K}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{Cr}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Sulphur}(\mathrm{Potassium}(\mathrm{Sulphuric}(\mathrm{Potassium}(\mathrm{Chromium}(\mathrm{Water}\left) \\ \mathrm{dioxide}\right)\mathrm{dichromate})\mathrm{acid})\mathrm{sulphate})\mathrm{sulphate}) \\ \left(\mathrm{orange}\right)\left(\mathrm{green}\right) \end{gathered}$$