Magnesium can reduce Al2O3 at a temperature below 1350∘C while above 1350∘C aluminium can reduce MgO because below 1350∘C∆G of MgO (formation) is more negative and above 1350∘C∆G of Al2O3 (formation) is more negative. The melting and boiling point of magnesium are lower than those of aluminium but it is not the correct reason.