CBSE Class 10 Science 2019 · Paper 31/5/2 · Question 8

Type of ore: a sulphide ore. The smell of rotten eggs is hydrogen sulphide gas, which a metal sulphide gives with dilute hydrochloric acid, for example $\text{ZnS}+2\text{HCl}⟶{\text{ZnCl}}_2+{\text{H}}_2\text{S}\uparrow$. Obtaining the metal from the concentrated ore (for a metal of medium reactivity such as zinc): (1) Roasting: the ore is heated strongly in excess air, which converts the sulphide into the oxide: $2\text{ZnS(s)}+3{\text{O}}_2\text{(g)}\stackrel{\text{heat}}{\to }2\text{ZnO(s)}+2{\text{SO}}_2\text{(g)}$. (2) Reduction: the oxide is heated with carbon (coke), which reduces it to the metal: $\text{ZnO(s)}+\text{C(s)}\stackrel{\text{heat}}{\to }\text{Zn(s)}+\text{CO(g)}$. (3) Refining: the impure metal is purified, for example by electrolytic refining. For a metal low in reactivity, such as mercury, roasting gives the oxide, which on further heating decomposes to the metal ($2\text{HgO}\stackrel{\text{heat}}{\to }2\text{Hg}+{\text{O}}_2$); this route also earns credit.