To identify the gas evolution reactions, we need to examine the products of each reaction to determine if any gases are produced. A gas evolution reaction is one in which at least one of the products is a gas. Let's analyze each reaction step-by-step:
1. [tex]$2 \text{HCl} (\text{aq}) + \text{K}_2\text{CO}_3 (\text{aq}) \rightarrow \text{H}_2\text{O} (\text{l}) + \text{CO}_2 (\text{g}) + 2 \text{KCl} (\text{aq})$[/tex]
Here, we see that one of the products is [tex]$\text{CO}_2 (\text{g})$[/tex], which is a gas. Hence, this reaction involves gas evolution.
2. [tex]$\text{CH}_4 (\text{g}) + 2 \text{O}_2 (\text{g}) \rightarrow \text{CO}_2 (\text{g}) + 2 \text{H}_2\text{O} (\text{l})$[/tex]
In this reaction, while [tex]$\text{CO}_2 (\text{g})$[/tex] is produced, both reactants and one of the products ([tex]$\text{CH}_4 (\text{g})$[/tex] and [tex]$\text{O}_2 (\text{g})$[/tex] as reactants) are already gases. This is a combustion reaction, and though it produces [tex]$\text{CO}_2 (\text{g})$[/tex], we are more interested in reactions where the evolution of gas is a signature effect (like in displacement reactions).
3. [tex]$2 \text{HCl} (\text{aq}) + \text{K}_2\text{S} (\text{aq}) \rightarrow \text{H}_2\text{S} (\text{g}) + 2 \text{KCl} (\text{aq})$[/tex]
Here, one of the products is [tex]$\text{H}_2\text{S} (\text{g})$[/tex], which is a gas. So, this reaction also involves gas evolution.
Thus, the reactions that involve the evolution of a gas as a significant product are:
- Reaction 1
- Reaction 3
Therefore, the correct selection is:
1 and 3 only
