Science — CBSE Class 10 board question

(i) Extraction of metals:

(I) Mercury from cinnabar (HgS):
Cinnabar (HgS) is heated in air. The ore gets converted to mercuric oxide, which on further heating decomposes to give mercury.

$$\text{HgS} + \text{O}_2 \xrightarrow{\Delta} \text{HgO} + \text{SO}_2$$
$$2\text{HgO} \xrightarrow{\Delta} 2\text{Hg} + \text{O}_2$$

(II) Copper from copper sulphide (Cu₂S):
Copper sulphide ore is heated in air to obtain copper directly.

$$\text{Cu}_2\text{S} + \text{O}_2 \xrightarrow{\Delta} 2\text{Cu} + \text{SO}_2$$

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(ii) Compounds formed as coating:

(I) Silver articles: A black coating of silver sulphide (Ag₂S) is formed when silver reacts with sulphur compounds present in air.

(II) Copper articles: A green coating of basic copper carbonate / copper carbonate [CuCO₃·Cu(OH)₂] is formed when copper reacts with CO₂ and moisture in air.

This process of metals being attacked by substances in their surroundings is called corrosion.

Source: Chapter 1, Chemical Reactions and Equations, Section on Corrosion

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• For (i), the examiner expects balanced equations with state symbols or at least correct formulae. Two-step extraction of mercury is important. For copper, a single-step roasting equation is standard at this level.
• For (ii), the exact names of the compounds are the key scoring points: Ag₂S (silver sulphide) for silver, and basic copper carbonate / Cu(OH)₂·CuCO₃ (green coating) for copper. Mentioning the term "corrosion" adds value. Avoid writing "rust" — rust is specifically for iron.