Question

Concentrated nitric acid renders iron passive. Why?

Answer 1

Iron will react with concentrated nitric acid, but not with highly concentrated nitric acid. That happens because very concentrated nitric acid will actually form a protective metal oxide layer on the surface of the metal, which essentially stops the reaction by preventing the further oxidation of the metal .

Answer 2

Concentrated nitric acid renders iron passive because of formation of reddish brown layer or coating of iron oxide which prevents its further reaction with the acid, hence nitric acid is helpful.