Question

Identify the gas from the following observations:

A colourless and odourless gas, which extinguishes a burning wooden splint. It turns lime-water milky, but has no action with potassium permanganate solution.

Answer 1

A colourless and odourless gas, which extinguishes a burning wooden splint is Carbon dioxide $\left({\mathrm{CO}}_2\right)$.

• To detect the odour of the gas, never bring the test tube close to your nose. Always make a waving motion with your hand to direct a vapours of the gas close to your nose. This will help you to ensure your safety in the chemistry laboratory.

This observation helps us in the qualitative analysis of gas.

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

Evolution of Carbon dioxide (CO₂):

Calcium carbonate (CaCO₃) reacts with dilute Hydrochloric acid (HCl) to produce Carbon dioxide (CO₂) along with Calcium chloride (CaCl₂) and water (H₂O).

• $$\begin{gathered} {\mathrm{CaCO}}_3\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Calcium}\mathrm{carbonate})(\mathrm{Hydrochloric}\mathrm{acid}))(\mathrm{Calcium}\mathrm{chloride})(\mathrm{Carbon}\mathrm{dioxide})(\mathrm{Water}) \end{gathered}$$

Carbon dioxide reacts with limewater to produce a white precipitate of Calcium carbonate (CaCO₃) which makes the solution look cloudy or milky in appearance.

• $$\begin{gathered} {\mathrm{CO}}_2\left(\mathrm{g}\right)+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Carbon}\mathrm{dioxide})(\mathrm{Calcium}\mathrm{hydroxide})(\mathrm{Calcium}\mathrm{carbonate})\left(\mathrm{Water}\right) \end{gathered}$$

Note that Sulphur dioxide $\left({\mathrm{SO}}_2\right)$ is also a colourless gas that turns limewater milky but it has pungent odour and reacts with Potassium permanganate (KMnO₄) and decolourises it but Carbon dioxide does not reacts with it.