Question

Give the chemical tests to distinguish between the following gases:

Sulphur dioxide and chlorine

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 Sulphur dioxide and Chlorine are as follows:

Name of the test	Sulphur dioxide $\left({\mathrm{SO}}_2\right)$	Chlorine $\left({\mathrm{Cl}}_2\right)$
Reaction with moist starch iodide paper	Does not turn moist starch iodide paper blue.	It turns moist starch iodide paper blue. $\underset{(\mathrm{Potassium}\mathrm{iodide})}{2\mathrm{KI}\left(\mathrm{aq}\right)}+\underset{\left(\mathrm{Chlorine}\right)}{{\mathrm{Cl}}_2\left(\mathrm{g}\right)}\to \underset{(\mathrm{Potassium}\mathrm{chloride})}{2\mathrm{KCl}\left(\mathrm{aq}\right)}+\underset{\left(\mathrm{Iodine}\right)}{{\mathrm{I}}_2\left(\mathrm{g}\right)}$$\mathrm{Starch}+\underset{\left(\mathrm{Iodine}\right)}{{\mathrm{I}}_2\left(\mathrm{g}\right)}\to \underset{(\mathrm{Blue}-\mathrm{black})}{\mathrm{Starch}-\mathrm{Iodine}\mathrm{complex}}$
Reaction with limewater.	Sulphur dioxide reacts with limewater to produce a white precipitate of Calcium sulphite (CaSO3) which makes the solution to look cloudy or milky in appearance. Limewater is an aqueous solution of Calcium hydroxide [Ca(OH)2]. $\underset{(\mathrm{Sulphur}\mathrm{dioxide})}{{\mathrm{SO}}_2\left(\mathrm{g}\right)}+\underset{(\mathrm{Calcium}\mathrm{hydroxide})}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}\to \underset{(\mathrm{Calcium}\mathrm{sulphite})}{{\mathrm{CaSO}}_3\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$	Chlorine gas does not turn limewater milky.