Passivity is the state where a metal becomes chemically unreactive despite its position in the activity series.

• Reaction with Iron: When Iron ($Fe$) is dipped in concentrated Nitric acid ($HN{O}_3$), which is a powerful oxidizing agent, it reacts briefly but then stops.
• Cause: The surface of the iron is oxidized to form an extremely thin, impervious, and protective oxide layer (specifically $F{e}_3{O}_4$).
• This layer acts as a physical barrier, preventing the acid from reaching the underlying bulk metal. In contrast, dilute $HN{O}_3$ dissolves iron normally as it doesn't form this protective passive layer.