Iron is rendered passive by the action of

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(D) Iron becomes passive when treated with concentrated $HNO_3$ due to the formation of a thin,protective,and non-porous oxide film $(Fe_2O_3)$ on its

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Conc. $HNO_3$

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(D) Iron becomes passive when treated with concentrated $HNO_3$ due to the formation of a thin,protective,and non-porous oxide film $(Fe_2O_3)$ on its surface. This layer prevents further reaction of the metal with the acid. Other oxidizing agents like $K_2Cr_2O_7$,$KMnO_4$,chloric acid,and chromic acid also render iron passive. Passive iron can be made active again by scratching the surface mechanically or by chemical reduction.

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