Supercooled water freezes at 263 K. Take Δ H = -6.01 kJ/mol and Δ Ssystem = -22.0 J/(K mol).

A spontaneous change is one that happens on its own. Entropy measures how spread out matter and energy are; gases have far more than liquids, and liquids more than solids. $\Delta S = q_{rev}/T$, so for melting or boiling $\Delta S = \Delta H/T$. The second law: $\Delta S_{total} = \Delta S_{sys} + \Delta S_{surr} > 0$ for a spontaneous change, with $\Delta S_{surr} = -\Delta H/T$. $\Delta_r S^\circ = \sum S^\circ(\text{products}) - \sum S^\circ(\text{reactants})$.