CBSE Class 10 Science 2020 · Paper 31/2/3 · Question 25(a)

Copper occurs in nature as sulphide ores such as copper glance, copper(I) sulphide (${\text{Cu}}_2\text{S}$). Pure copper is obtained from copper glance in this sequence: Step 1: Concentration (enrichment) of the ore: the gangue, such as soil and sand, is removed (for a sulphide ore, by froth flotation). Step 2: Roasting: the concentrated ore is heated strongly in air, and part of the copper(I) sulphide is converted into copper(I) oxide: $2{\text{Cu}}_2\text{S}+3{\text{O}}_2\stackrel{\text{heat}}{\to }2{\text{Cu}}_2\text{O}+2{\text{SO}}_2$ Step 3: Self-reduction: on continued heating, the copper(I) oxide reacts with the remaining copper(I) sulphide to give copper: $2{\text{Cu}}_2\text{O}+{\text{Cu}}_2\text{S}\stackrel{\text{heat}}{\to }6\text{Cu}+{\text{SO}}_2$ Step 4: Electrolytic refining: the copper so obtained is impure. It is refined in an electrolytic cell with a thick block of impure copper as the anode, a thin strip of pure copper as the cathode, and a solution of copper sulphate acidified with dilute sulphuric acid as the electrolyte. On passing current, copper from the anode dissolves into the electrolyte, $\text{Cu}⟶{\text{Cu}}^{2+}+2e^{-}$, and an equivalent amount of pure copper from the electrolyte is deposited on the cathode, ${\text{Cu}}^{2+}+2e^{-}⟶\text{Cu}$. Soluble impurities go into the solution, and insoluble impurities settle at the bottom below the anode as anode mud.