Explain the type of reactions represented by the following equations:
$(i)$ $Mg + CuSO_4 \longrightarrow MgSO_4 + Cu$
$(ii)$ $NH_4NO_2 \longrightarrow N_2 + 2H_2O$

(N/A) $(i)$ Displacement reaction: $A$ more reactive metal $(Mg)$ displaces a less reactive metal $(Cu)$ from its salt solution $(CuSO_4)$.
$(ii)$ Decomposition reaction: $A$ single compound $(NH_4NO_2)$ breaks down into two or more simpler substances ($N_2$ and $H_2O$).