CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 25(a)

(a) Metals in the middle of the reactivity series (zinc, iron, lead, copper) usually occur as sulphides or carbonates; the ore is first converted into the oxide, by roasting a sulphide or calcining a carbonate, and the oxide is then reduced to the metal by heating it with carbon. Metals high up in the series (potassium, sodium, calcium, magnesium, aluminium) cannot be obtained this way: they are extracted by electrolytic reduction of their molten chlorides or oxides. The carbon process cannot be applied to them because these metals have a greater affinity for oxygen than carbon has, so carbon cannot reduce their oxides. Extraction of sodium is by electrolytic reduction: electricity is passed through molten sodium chloride; sodium metal is deposited at the cathode and chlorine gas is liberated at the anode. At the cathode: ${\text{Na}}^{+}+e^{-}⟶\text{Na}$ At the anode: $2{\text{Cl}}^{-}⟶{\text{Cl}}_2+2e^{-}$ (b) Labelled diagram (see the figure): a tank of acidified copper sulphate solution as the electrolyte, a block of impure copper as the anode (+), a thin strip of pure copper as the cathode (−), and the anode mud of insoluble impurities below the anode; copper dissolves from the anode and pure copper is deposited on the cathode.