Concentrated nitric acid is an oxidising agent and attacks most metals except noble metals such as gold and platinum. The products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation.
Zn + 4HNO3(conc.)→ Zn(NO3)2 + 2H2O + 2NO2
Zinc nitrate, water, and nitrogen dioxide (brown gas) are the by-product of this reaction.
This brown gas (nitrogen dioxide) dimerise on cooling to form nitrogen tetraoxide which is stable than the later one.
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