In the given chemical reaction:
[tex]\[
Fe (s) + CuSO_4 (aq) \rightarrow Cu (s) + FeSO_4 (aq)
\][/tex]
we need to identify the reactant that is dissolved in solution. To do this, we observe the states of the reactants and products:
- \(Fe (s)\) is a solid.
- \(CuSO_4 (aq)\) is an aqueous compound, which means it is dissolved in water.
- \(Cu (s)\) is a solid.
- \(FeSO_4 (aq)\) is an aqueous compound, which means it is dissolved in water.
Since we are interested in the reactants only, we look at \(Fe (s)\) and \(CuSO_4 (aq)\).
Out of these:
- \(Fe (s)\) is a solid and not dissolved in solution.
- \(CuSO_4 (aq)\) is in the aqueous state, indicating it is dissolved in solution.
Thus, the reactant that is a substance dissolved in solution is:
[tex]\[ \boxed{CuSO_4} \][/tex]
