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Copper reacts differently with dilute and concentrated nitric acid due to the varying oxidizing power of the acid:
Dilute Nitric Acid: Copper reacts with dilute nitric acid to produce copper(II) nitrate ($Cu(NO_3)_2$), nitric oxide ($NO$) gas, and water. The nitric acid acts as an oxidizing agent, oxidizing copper to copper(II) ions. The reaction is:
3Cu(s) + 8HNO₃(aq) → 3Cu(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)
Concentrated Nitric Acid: In concentrated nitric acid, copper reacts to produce copper(II) nitrate, nitrogen dioxide ($NO_2$) gas, and water. The concentrated acid is a stronger oxidizing agent, leading to the formation of nitrogen dioxide instead of nitric oxide. The reaction is:
Cu(s) + 4HNO₃(aq) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)