When copper is heated with concentrated nitric acid ($\text{HNO}_3$), it undergoes oxidation to form copper(II) nitrate, while the nitric acid acts as an oxidizing agent and is reduced to nitrogen dioxide ($\text{NO}_2$) gas. The balanced chemical equation for the reaction is: $\text{Cu} + 4\text{HNO}_3 \text{ (conc.)} \rightarrow \text{Cu}(\text{NO}_3)_2 + 2\text{NO}_2 \uparrow + 2\text{H}_2\text{O}$ Note: If dilute $\text{HNO}_3$ is used, it produces nitric oxide ($\text{NO}$) instead of $\text{NO}_2$.