Question

why Cr,Al do not dissolve in conc.HNO3???

Answer 1

Due to great affinity for O2 , they get oxidized to form their respective kxides which forms a layer over the metal surfaces that prevents reaction......

Answer 2

Answer:

• Nitric acid is a strong oxidizing agent. When Cr or Al are added to $HNO_{3}$, they oxidise to form chromium oxide and Aluminum oxide. These metals undergo surface oxidation due to which a film of oxide on the surface of nitric acid is formed.

• Due to oxidation they are passive in the concentrated acid which doesn't allow the metals to dissolve in it.

• They don't get dissolved but instead they get passivated.

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