When a strong acid like nitric acid ($HNO_3$) is added to the tetraamminecopper(II) complex $[Cu(NH_3)_4]^{2+}$, the acid protonates the basic ammonia ($NH_3$) ligands to form ammonium ions ($NH_4^+$). This breaks the complex, releasing $Cu^{2+}$ ions into the aqueous solution. The $Cu^{2+}$ ions combine with the available nitrate ($NO_3^-$) ions from the acid to form copper(II) nitrate. The reaction can be summarised as: $[Cu(NH_3)_4]^{2+} + 4HNO_3 ightarrow Cu(NO_3)_2 + 4NH_4^+ + 2NO_3^-$. Therefore, aqueous $Cu(NO_3)_2$ is obtained.