Question

Iron is rendered passive by treatment with concentrated

• A. $H_2SO_4$
• B. $H_3PO_4$
• C. HCl
• D. $HNO_3$

Answer 1

Answer: (D)

$HNO_3$

Answer 2

Iron is rendered passive by concentrated $HNO_3$ due to formation of a thick protective layer of $Fe_3O_4$.