CBSE Class 10 Science 2019 · Paper 31/2/2 · Question 4

(a) It is easier to obtain a metal from its oxide than from its carbonate or sulphide, because the oxide can be reduced to the metal directly by heating it with carbon. So a sulphide ore is roasted (heated strongly in excess air) and a carbonate ore is calcined (heated strongly in limited air) to change it into the oxide first: $2\text{ZnS}+3{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{ZnO}+2{\text{SO}}_2$, ${\text{ZnCO}}_3\stackrel{\text{heat}}{\to }\text{ZnO}+{\text{CO}}_2$; then $\text{ZnO}+\text{C}⟶\text{Zn}+\text{CO}$. (b) Aluminium reacts with the oxygen of the air to form a thin, tough, protective layer of aluminium oxide on its surface (made thicker by anodising). This layer does not let air and water reach the metal, so it does not react further or corrode. Being light and a good conductor of heat, aluminium is therefore widely used for cooking utensils.