On heating Al at 800° C in air, Al₂O₃ is formed. The reaction is

The oxidation of aluminum to form aluminum oxide is an exothermic reaction because it releases heat energy. When aluminum reacts with oxygen, it undergoes a highly exothermic reaction that produces aluminum oxide and a large amount of heat.

$2{\mathrm{Al}}_{\left(\mathrm{s}\right)}+3{\mathrm{O}}_{2_{\left(\mathrm{g}\right)}}\to 2{\mathrm{Al}}_2{\mathrm{O}}_{3_{\left(\mathrm{s}\right)}}+\mathrm{Heat}$