What is the observation made when conc. nitric acid is added to copper turnings?

When concentrated nitric acid is added to copper turnings, a chemical reaction occurs where the copper is oxidized by the nitric acid. The observation made during this reaction is the evolution of brown nitrogen dioxide gas, along with the formation of a blue solution of copper(II) nitrate. The copper turnings dissolve in the nitric acid, forming a blue-green solution.

When concentrated nitric acid is added to copper turnings, a vigorous reaction takes place and a brown gas, namely nitrogen dioxide, is evolved. This is because copper reacts with nitric acid to form copper(II) nitrate and nitrogen dioxide gas. The reaction is highly exothermic, producing a lot of heat.