To determine which substance is the acid in the reaction, we need to understand the behavior of each substance in this chemical equation and recognize the defining characteristics of an acid.
The reaction given is:
[tex]\[ H_2SO_4(aq) + Mg(s) \longrightarrow MgSO_4(aq) + H_2(g) \][/tex]
We consider the definitions and properties of acids:
1. Acids are substances that donate protons (H⁺ ions) in an aqueous solution.
2. Bases are substances that accept protons.
Now, let's analyze each substance:
- [tex]\( Mg (s) \)[/tex]: This is solid magnesium. Metals typically do not donate H⁺ ions in aqueous solutions, thus magnesium is not an acid in this context.
- [tex]\( H_2(g) \)[/tex]: This is hydrogen gas. Hydrogen gas is a diatomic molecule and does not donate H⁺ ions; thus, it is not an acid.
- [tex]\( MgSO_4(aq) \)[/tex]: This is magnesium sulfate in aqueous solution. Magnesium sulfate is a salt, and salts do not typically act as acids; they do not donate H⁺ ions.
- [tex]\( H_2SO_4(aq) \)[/tex]: This is sulfuric acid in an aqueous solution. Sulfuric acid is a well-known strong acid that dissociates in water to give H⁺ ions. Given that it donates H⁺ ions in an aqueous solution, it fits the definition of an acid.
Considering these observations, the substance that acts as an acid in this reaction is:
[tex]\[ H_2SO_4(aq) \][/tex]
Therefore, the acid in the reaction is [tex]\( H_2SO_4(aq) \)[/tex], which corresponds to the fourth substance:
[tex]\[
\boxed{4}
\][/tex]
