Here's a breakdown:

* Copper (Cu) is oxidized, meaning it loses electrons and changes from its elemental state to a +2 oxidation state (Cu²⁺).

* Nitric acid (HNO₃) is reduced, meaning it gains electrons. The nitrate ion (NO₃⁻) is reduced to either nitrogen dioxide (NO₂) or nitrogen monoxide (NO) depending on the concentration of nitric acid.

The overall reaction:

* Concentrated HNO₃:

Cu(s) + 4HNO₃(aq) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

* Dilute HNO₃:

3Cu(s) + 8HNO₃(aq) → 3Cu(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)

Key observations:

* Brown gas: The reaction releases brown fumes of nitrogen dioxide (NO₂) when concentrated nitric acid is used, or colorless nitrogen monoxide (NO) when dilute acid is used.

* Blue solution: Copper nitrate (Cu(NO₃)₂) dissolves in the acid, creating a blue solution.

* Heat: The reaction is exothermic and generates heat.

Why this is a redox reaction:

The reaction involves the transfer of electrons from copper to nitric acid. This change in oxidation states is the defining characteristic of a redox reaction.