In which of the following changes, entropy of the system decreases?

Entropy is a measure of the randomness or disorder of a system.

For the dissolution of solid $\text{I}_2$ in water, a solid changes into an aqueous solution, which increases the randomness. Thus, entropy increases ($\Delta S > 0$).

For the dissociation of $\text{H}_2$ molecules into atoms, $\text{H}_2(g) \rightarrow 2\text{H}(g)$, the number of gaseous particles increases. Thus, entropy increases ($\Delta S > 0$).

For the formation of liquid water from hydrogen and oxygen gas molecules, $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$, gaseous reactants are converted into a liquid product. The number of moles of gas decreases ($\Delta n_g = -3$), leading to a decrease in randomness. Thus, entropy decreases ($\Delta S < 0$).

For the melting of ice at 0^{}^{\circ}\text{C}, a solid changes into a liquid, which increases the randomness. Thus, entropy increases ($\Delta S > 0$).

Answer: Formation of liquid water from hydrogen and oxygen gas molecules