Question

Identify the gas from the following observations:

A colourless gas having a rotten eggs like smell. It turns limewater milky and moist lead acetate paper silvery black.

Answer 1

Hydrogen sulphide gas (H₂S) is a colourless gas that has the smell of rotten eggs. This observation acts as a preliminary test in the qualitative analysis of Hydrogen sulphide.

• Qualitative analysis: The analysis which aims at detecting the presence of any substance in a given sample. Qualitative analysis is usually done in two steps.

1. Preliminary tests are the initial tests that give us an idea of a substance. These tests include observations like the colour, smell and physical state of any substance. These tests give us a hint of what an unknown substance might be.
2. Confirmatory tests are the tests that follow a preliminary test. These tests include the chemical reactions that help us to confirm that chemical substance.

In the lab, Hydrogen sulphide is produced by reacting a metal sulphide (Ferrous sulphide) with a dilute acid (Sulphuric acid).

$$\begin{gathered} \mathrm{FeS}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{FeSO}}_4\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right) \\ (\mathrm{Ferrous}\mathrm{sulphide})(\mathrm{Sulphuric}\mathrm{acid})(\mathrm{Ferrous}\mathrm{sulphate})(\mathrm{Hydrogen}\mathrm{sulphide}) \end{gathered}$$

The element responsible for the rotten egg-like smell of Hydrogen sulphide is Sulphur (S).

• Hydrogen sulphide (H₂S) gas turns lead acetate paper black.
• Lead acetate is (CH₃COO)₂Pb.

$$\begin{gathered} {\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Pb}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)\to \mathrm{PbS}\left(\mathrm{s}\right)+2{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right) \\ (\mathrm{Lead}\mathrm{acetate})(\mathrm{Hydrogen}\mathrm{sulphide})(\mathrm{Lead}\mathrm{sulphide})(\mathrm{Acetic}\mathrm{acid}) \\ \left(\mathrm{black}\right) \end{gathered}$$

A filter paper is dipped in Lead acetate solution and it turns black due to the formation of Lead sulphide which is black in colour.