CBSE Class 10 Science 2018 · Paper 31/1 · Question 16

(a) After the ore is concentrated, a metal in the middle of the activity series is extracted from its carbonate ore in three steps, in this order: (i) Calcination: the carbonate ore is heated strongly in limited air and changes into the metal oxide, for example ${\text{ZnCO}}_3\stackrel{\text{heat}}{\to }\text{ZnO}+{\text{CO}}_2$. (ii) Reduction: the metal oxide is heated with a reducing agent such as carbon, for example $\text{ZnO}+\text{C}\to \text{Zn}+\text{CO}$. (iii) Purification (refining) of the impure metal, usually by electrolytic refining. (b) Copper glance, ${\text{Cu}}_2\text{S}$, is heated in air (roasting), and part of it is changed into copper(I) oxide: $2{\text{Cu}}_2\text{S}+3{\text{O}}_2\stackrel{\text{heat}}{\to }2{\text{Cu}}_2\text{O}+2{\text{SO}}_2$. On further heating, the oxide reacts with the remaining sulphide and copper is formed (self-reduction): $2{\text{Cu}}_2\text{O}+{\text{Cu}}_2\text{S}\stackrel{\text{heat}}{\to }6\text{Cu}+{\text{SO}}_2$. The impure copper is refined electrolytically (labelled diagram in the figure): the electrolyte is acidified copper sulphate solution, the anode is a block of impure copper connected to the positive terminal, and the cathode is a thin strip of pure copper connected to the negative terminal. Copper dissolves from the anode, $\text{Cu}\to {\text{Cu}}^{2+}+2e^{-}$, and pure copper is deposited on the cathode, ${\text{Cu}}^{2+}+2e^{-}\to \text{Cu}$; the insoluble impurities settle below the anode as anode mud.