Question

Give one example of a double decomposition reaction that is accompanied by the evolution of gas..

Also, suggest one chemical test to identify the evolved gas.

Write relevant chemical reactions in support of your answer.

Answer 1

1. Double displacement or partner exchange reaction is one in which two ionic compounds form two other ionic compounds when cations and anions of the two compounds in the reactant swap place producing new compounds.
2. The symbolic representation of the double displacement reaction is as follows:

$AB+CD\to AD+CB$

Here $AandC$ are cations, and $BandD$ are anions.

An example of a double displacement reaction in which gas is evolved is as follows:

$\underset{(Sodiumsulphide)}{\underbrace{N{a}_{2}S\left(s\right)}}+\underset{(Hydrochloricacid)}{\underbrace{2HCl\left(aq\right)}}\to \underset{(Sodiumchloride)}{\underbrace{2NaCl\left(aq\right)}}+\underset{(Hydrogensulphide)}{\underbrace{H_{2}S\left(g\right)\uparrow }}$

In the above double displacement reaction, Hydrogen sulfide is forming.

Test for Hydrogen sulfide would be,

1. The gas is colorless with a foul smell like rotten eggs.
2. It turns moist blue litmus red.
3. It turns lead acetate paper black.
4. It also turns the lead nitrate solution black.