Give reasons for the following: Zinc oxide can be reduced to metal by heating with carbon but Cr2O3 cannot be reduced by heating with carbon.

Because carbon has a higher affinity for oxygen than zinc, oxides of moderately reactive metals such as Zn can be reduced by carbon, resulting in ZnO being reduced to Zn and carbon combining with oxygen to form CO. Whereas chromium oxides cannot be reduced by carbon because carbon has a lower affinity for oxygen than chromium, due to which chromium oxide cannot be reduced by carbon.