NH4NO3 (aq) → N2O (g) + 2 H2O (l)
This is an oxidation-reduction (redox) reaction:
N-III - 4 e- → NI (oxidation)
NV + 4 e- → NI (reduction)
NH4NO3 is a reducing agent, NH4NO3 is an oxidizing agent (synproportionation (comproportionation)).
This is a gas evolution reaction, N2O is the formed gas.