It can be observed from the above graph that a metal can reduce the oxide of other metals, if the standard free energy of formation Δ_fG^ø of the oxide of the former is more negative than the latter. For example, since Δ_fG^ø_(Al,Al2,O3) is more negative than Δ_fG^ø_(Cu,Cu2,O) , Al can reduce Cu₂O to Cu, but Cu cannot reduce Al₂O₃. Similarly, Mg can reduce ZnO to Zn, but Zn cannot reduce MgO because Δ_fG^ø_(Mg,MgO) is more negative thanΔ_fG^ø_(Zn,ZnO) .