decomposition reaction
Yes, it can be. Example: Fe + CuSO4 ----------> FeSO4 + Cu Here Fe is oxidized to Fe2+ and Cu2+ is reduced to Cu.
decomposition
Yes it can. I believe that an example of such a reaction would be the addition of zinc to a solution of 6 molar hydrochloric acid (HCl).
Synthesis reactions are generally characterized by being exothermic. Decomposition reactions are usually endothermic.