Science — CBSE Class 10 board question

(i)

(I) Heating a mixture of Cu₂O and Cu₂S:

$$2\text{Cu}_2\text{O} + \text{Cu}_2\text{S} \rightarrow 6\text{Cu} + \text{SO}_2\uparrow$$

(II) ZnS heated in the presence of oxygen:

$$2\text{ZnS} + 3\text{O}_2 \xrightarrow{\Delta} 2\text{ZnO} + 2\text{SO}_2$$

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(ii)

(I) PVC (polyvinyl chloride) is a poor conductor of electricity (insulator). It prevents electric shock to persons handling the wires.

(II) Copper does not react with hot water and does not corrode easily, making it suitable for hot water tanks. Steel (iron) reacts with hot water/steam and corrodes, contaminating water.

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(iii) Formation of CaO:

Ca has 2 valence electrons; O needs 2 electrons to complete its octet.

$$\text{Ca} [\cdot\cdot] + [\cdot\cdot\underset{\cdot\cdot}{\text{O}}\cdot\cdot] \rightarrow \text{Ca}^{2+}[\underset{\cdot\cdot}{\ddot{\text{O}}}\underset{\cdot\cdot}{}]^{2-}$$

Ca donates 2 electrons to O. Ca becomes Ca²⁺ and O becomes O²⁻, forming the ionic compound CaO.

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• Q(i): These are standard metallurgy reactions. The Cu₂O–Cu₂S reaction is the self-reduction used in copper extraction. ZnS roasting converts sulphide ore to oxide — a key step in metallurgy.
• Q(ii): For PVC coating, the keyword is "insulator/poor conductor → prevents electric shock." For copper tanks, focus on resistance to corrosion and non-reactivity with hot water. Avoid vague answers like "copper is better."
• Q(iii): Draw the electron dot (Lewis) structure clearly. Show electron transfer: Ca loses 2e⁻ → Ca²⁺; O gains 2e⁻ → O²⁻. Mention ionic bond formation. Even a rough diagram with dots/crosses earns the mark if the transfer is correctly shown.