A) Conc. \[{{H}_{2}}S{{O}_{4}}\]
B) Conc. \[{{H}_{3}}P{{O}_{4}}\]
C) Conc. \[HCl\]
D) Conc. \[HN{{O}_{3}}\]
Correct Answer: D
Solution :
Iron is rendered passive by conc. \[HN{{O}_{3}}\] and other oxidising agents like \[{{K}_{2}}C{{r}_{2}}{{O}_{7}},\,KMn{{O}_{4}}\], Chloric acid, chromic acid, silver nitrate etc. A specimen of passive iron can be rendered active by scratching the film mechanically or chemically.
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