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Iron is rendered passive when treated with ____, which works by producing a ____ on the metal surface.

1. dil. HCl, Fe2O3
2. 80% conc. HNO3, Fe3O4
3. conc. H2SO4, Fe3O4
4. conc. HCl, Fe3O4

Correct answer: 80% conc. HNO3, Fe3O4

Solution

Concentrated nitric acid oxidises the iron surface to a thin, dense, adherent oxide layer (Fe3O4) that prevents further chemical attack, making the metal passive.

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