What is the primary gas evolved when concentrated nitric acid reacts with copper metal?

💡 Solution & Explanation

When copper metal reacts with concentrated nitric acid, it undergoes oxidation to form copper nitrate while reducing the acid to produce dense brown fumes of nitrogen dioxide gas. Weaker or dilute nitric acid reactions with copper tend to yield colorless nitric oxide instead of nitrogen dioxide. This reaction demonstrates the strong oxidizing property of concentrated nitric acid. A related laboratory observation is that the solution turns blue upon the formation of hydrated copper ions.