Science (086) — CBSE Class 10 board question

(i) Mercury – Mercury is low in the activity series. HgS (cinnabar) is first roasted in air to form HgO, which on further heating is reduced to mercury by thermal decomposition (heating alone):
$$2\text{HgO} \xrightarrow{\Delta} 2\text{Hg} + \text{O}_2$$

(ii) Copper – Copper is also low in the activity series. Cu₂S is heated in air; the Cu₂O formed reacts with remaining Cu₂S to give copper by thermal reduction (heating alone):
$$2\text{Cu}_2\text{O} + \text{Cu}_2\text{S} \xrightarrow{\Delta} 6\text{Cu} + \text{SO}_2$$

(iii) Sodium – Sodium is at the top of the activity series and is highly reactive. Carbon cannot reduce its oxide. So sodium is obtained by electrolytic reduction of molten NaCl:
$$\text{Na}^+ + e^- \rightarrow \text{Na (at cathode)}$$

Source: Chapter 3, Sections 3.4.3 and 3.4.5

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• The key to this question is linking reactivity position to extraction method: low reactivity → heating alone; high reactivity → electrolysis.
• Always state the reason (position in activity series) before the method — examiners look for it explicitly.
• Writing the chemical equation earns marks and shows precision; include it for each metal.
• Sodium cannot be reduced by carbon because it has greater affinity for oxygen than carbon does — this is the reason the examiner wants.