Question

Give the chemical tests to distinguish between the following gases:

Hydrogen sulphide and carbon dioxide

Answer 1

In practical Chemistry, two types of analysis are done.

1. Qualitative analysis: The analysis which aims at detecting the presence of any substance in a given sample.
2. Quantitative analysis: The analysis which aims at knowing the quantity of any substance present in a sample.

These chemical tests include the chemical reactions of the gases with other chemicals that help us to confirm the gas by qualitative analysis.

• Physical tests like colour and odour are also helpful but do not help to confirm the gas.
• Chemical tests to distinguish between Hydrogen sulphide and Carbon dioxide are as follows:

Name of the test	Hydrogen sulphide $\left({\mathrm{H}}_2\mathrm{S}\right)$	Carbon dioxide $\left({\mathrm{CO}}_2\right)$
Reaction with moist lead acetate paper	Hydrogen sulphide (H2S) gas turns lead acetate paper black. $\underset{(\mathrm{Lead}\mathrm{acetate})}{{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Pb}\left(\mathrm{aq}\right)}+\underset{(\mathrm{Hydrogen}\mathrm{sulphide})}{{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)}\to \underset{\underset{\left(\mathrm{black}\right)}{(\mathrm{Lead}\mathrm{sulphide})}}{\mathrm{PbS}\left(\mathrm{s}\right)}+\underset{(\mathrm{Acetic}\mathrm{acid})}{2{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)}$	Carbon dioxide does not turn Lead acetate paper black.
Reaction with limewater	It does not turn limewater milky.	Carbon dioxide reacts with limewater to produce a white precipitate of Calcium carbonate (CaCO3) which makes the solution to look cloudy or milky in appearance$\underset{(\mathrm{Carbon}\mathrm{dioxide})}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{(\mathrm{Calcium}\mathrm{hydroxide})}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}\to \underset{(\mathrm{Calcium}\mathrm{carbonate})}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$