When steam is passed over red-hot iron, iron reacts with water vapor to form iron oxide and hydrogen gas. The balanced chemical equation for this reaction is:
\[
3Fe (s) + 4H_2O (g) \rightarrow Fe_3O_4 (s) + 4H_2 (g)
\]
In this reaction, iron (Fe) reacts with steam (water vapor, H2O) to form iron oxide (Fe3O4) and hydrogen gas (H2).