reaction of nitric acid with metals show different results, why?

it all depends on the conc of the nitric acid and the nature of the metal to some extent.

for eg 

1 if nitric acid is conc then remember only nitrogen dioxide is formed.

2 if the nitric acid is very dilute then dinitrogen oxide n[sub]2[/sub]o is formed

thus in this way you can say that nitric acid has different reaction with different metals.

Regards,

gp

nitric acid acts as a strong oxidizing agent as well as an acid. Reactive metals will react with nitric acid to give the metal nitrate and hydrogen gas. Less reactive metals do not produce hydrogen at all. With dilute acid one of the principal products is nitrogen monoxide, while with concentrated acid it is nitrogen dioxide.

both reactions give the brown fumes normally associated with nitrogen dioxide, because the nitrogen monoxide rapidly reacts with the oxygen in the air to produce nitrogen dioxide. Treating metals (iron, chromium, aluminum, and calcium) with concentrated nitric acid can produce a protective oxide layer over the metal that halts further reactions.