Question

Give a chemical test to distinguish between the following pair of compounds:
Hydrogen chloride gas and hydrogen sulfide gas.

Answer 1

The chemical formula for hydrogen chloride gas is $HCl$ , and the chemical formula for hydrogen sulfide gas is $H_2S$. The two compounds can be distinguished chemically by testing with a solution of ammonia.

Complete step by step answer:
First, let us take a test tube containing a solution ammonia and a glass rod. Dip the glass rod in the solution and introduce it to a test tube containing hydrogen chloride gas. On doing so, we immediately observe dense white fumes appearing. These white fumes are those of ammonium chloride $(NH_4Cl)$. The chemical reaction between hydrogen chloride gas and ammonia solution can be represented through the following equation:
$HCl(g) + NH_3(aq.) \rightarrow NH_4Cl(g)$
On the other hand, when a rod dipped in ammonia solution is held near the mouth of a test tube containing hydrogen sulfide gas, then no such fumes appear. This is because of the formation of ammonium hydrogen sulfide, having the chemical formula $(NH_4)SH$, instead of ammonium chloride as in the case of hydrogen chloride. The reaction between hydrogen sulfide gas and ammonia solution can be represented chemically as follows:
$H_2S(g) + NH_3(aq.) \rightarrow (NH_4)SH(g)$

Note: Hydrogen chloride is a yellowish-green colored gas with a pungent odor.
Hydrogen sulfide is a black colored gas with an unpleasant smell of rotten eggs.
Hydrogen chloride gas and hydrochloric acid, both have the formula of $HCl$. But, one should not be confused with the other. When hydrogen chloride gas is mixed with water, the solution forms is called hydrochloric acid, which is a colorless liquid.
The ammonia must be dissolved in water in order to distinguish between hydrogen chloride and hydrogen sulfide gases. The reactions cannot be carried out with gaseous ammonia.