Freezing
The fact that water has previously been warmed contributes to its freezing quickly. ~ Aristotle
Water nominally freezes at 0 °C, but, like most other liquids, may exist as a supercooled liquid: staying fluid below its freezing point. Liquid water at –40 °C has been found in clouds. Water may remain a liquid to –48 °C. Supercooled water freezes if disturbed.
The freezing of water is controlled not only by its temperature, but also by its size. The nucleation of ice in small droplets is strongly size-dependent.
Freezing is a process of water molecules forming a bonded network. In transition to freezing, water physically changes into tetrahedrons, with each water molecule loosely bonded to 4 others.
Freezing water droplets garb themselves with pointy tips, sporting fractal shapes as they crystallize. Depending upon temperature and pressure, ice may take various crystalline forms while water molecules cling to each other via hydrogen bonds.
Aristotle, and others after him, noted that hot water freezes faster than cold water. Yet namesake credit is given for its 1963 rediscovery by Tanzanian scientist Erasto Mpemba, who first noticed the Mpemba effect when he was 13, while making ice cream in haste: mixing boiling milk with sugar, rather than waiting for the milk to cool first, as instructed.
Mpemba asked his physics teacher for an explanation. The teacher told Mpemba that he must have been confused, as what he supposedly saw was impossible. Mpemba persisted in his inquiry.
Though the mechanics of the Mpemba effect remain something of a mystery, the likely explanation lies in the strength and arrangement of hydrogen bonds, which are affected by temperature, and partly depend upon nearby water molecules.
