During which of the following processes does entropy decrease? (A) Freezing of water to ice at 0C (B) Freezing of water…

🧠 Entropy decreases when disorder decreases — fewer gas moles, more order, or adsorption Check each process for whether the system becomes more ordered or less.

🗺️ Checking each process (A) Freezing at 0°C: liquid → solid, more ordered → $\Delta S < 0$ ✓

(B) Freezing at −10°C: same, also spontaneous below 0°C → $\Delta S_{\text{sys}} < 0$ ✓

(C) $\ce{N2 + 3H2 -> 2NH3}$: 4 mol gas → 2 mol gas, $\Delta n_g = -2$ → $\Delta S < 0$ ✓

(D) Adsorption of CO(g) on lead surface: gas molecules become fixed on a surface — huge decrease in freedom → $\Delta S < 0$ ✓

(E) Dissolution of NaCl in water: solid breaks into freely moving ions, increases disorder → $\Delta S > 0$ ✗

Processes A, B, C, D all decrease entropy → option (a).

⚠️ Trap: Dissolution of NaCl is endothermic AND increases entropy. Many students confuse "dissolves with difficulty" (endothermic) with "decreases entropy" — these are unrelated.

$\boxed{\text{Answer: (a)}}$