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In which of the following processes does the entropy of the system decrease?

A $H X_{2} O (g) H X_{2} O (l)$ at $298 K$
B $H X_{2} O (s) H X_{2} O (l)$ at $273 K$
C $NH X_{4} Cl (s) NH X_{3} (g) + HCl (g)$
DExpansion of an ideal gas into vacuum

Answer: A

Condensation of vapour to liquid reduces the molecular disorder and the number of accessible microstates, so

Δ S_{sys} < 0

. Melting, decomposition into gases and free expansion all increase disorder.

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