(a) State giving reason the reduction process to obtain the following metals from their compounds: (i) Mercury, (ii) Copper and (iii) Sodium. OR (b) State giving reason for the change in appearance observed when each of the following metal is exposed to atmospheric air for some time: (i) Silver, (ii) Copper and (iii) Iron.

Option (a)

#### (i) Mercury

• Reduction Process: Heating alone (Thermal decomposition).
• Reason: Mercury is a metal of very low reactivity. Its oxide ($HgO$) is thermally unstable and can be reduced to metallic mercury simply by heating it strongly in air.
• Chemical Equation:

$$2HgO(s) \xrightarrow{\text{Heat}} 2Hg(l) + O_2(g)$$

#### (ii) Copper

• Reduction Process: Heating alone in air (Self-reduction).
• Reason: Copper is low in the reactivity series. Its sulphide ore (copper glance, $Cu_2S$) is first heated in air to convert it partially into copper oxide ($Cu_2O$). On further heating in the absence of excess air, the oxide reacts with the remaining sulphide to yield copper metal.
• Chemical Equations:

$$2Cu_2S(s) + 3O_2(g) \xrightarrow{\text{Heat}} 2Cu_2O(s) + 2SO_2(g)$$

$$2Cu_2O(s) + Cu_2S(s) \xrightarrow{\text{Heat}} 6Cu(s) + SO_2(g)$$

#### (iii) Sodium

• Reduction Process: Electrolytic reduction (Electrolysis of molten sodium chloride).
• Reason: Sodium is highly reactive and placed at the top of the reactivity series. It has a very high affinity for oxygen, making chemical reduction with carbon (coke) impossible. Therefore, it is extracted by passing electricity through molten $NaCl$.
• Electrode Reactions:
• At Cathode (Reduction): $$Na^+ + e^- \rightarrow Na(s)$$
• At Anode (Oxidation): $$2Cl^- \rightarrow Cl_2(g) + 2e^-$$

---

OR

Option (b)

#### (i) Silver

• Change in Appearance: Silver articles lose their shine and turn black.
• Reason: Silver reacts with hydrogen sulphide ($H_2S$) gas present in the atmosphere to form a thin, black layer of silver sulphide ($Ag_2S$).
• Chemical Equation:

$$2Ag(s) + H_2S(g) \rightarrow Ag_2S(s) + H_2(g)$$

#### (ii) Copper

• Change in Appearance: Copper loses its shiny brown surface and develops a green coating.
• Reason: Copper reacts slowly with moist carbon dioxide ($CO_2$), oxygen, and water vapor in the air to form a green layer of basic copper carbonate [$$CuCO_3 \cdot Cu(OH)_2$$].

#### (iii) Iron

• Change in Appearance: Iron develops a rough, flaky, reddish-brown coating.
• Reason: When exposed to moist air (containing both oxygen and water) for a long time, iron undergoes oxidation and hydration to form a reddish-brown substance called rust (hydrated iron(III) oxide, $$Fe_2O_3 \cdot xH_2O$$).