Pure water can stay liquid far below zero, down to about −48°C.
Water does not freeze just because it hits 0°C. Ice needs a seed to start growing on, such as a speck of dust or a scratch. Take the seeds away and water stays liquid far below freezing, then turns to ice in seconds the moment it is disturbed.
Freezing is not automatic. For liquid water to become ice, its molecules have to begin arranging into a crystal, and that usually starts on something: a dust particle, an impurity, a rough patch on a container, or an existing ice crystal. Without such a nucleus, a liquid can be cooled below its normal freezing point and stay liquid. This is supercooling, first described in 1724 by Daniel Gabriel Fahrenheit while he was developing his temperature scale.
Purified water can be supercooled at ordinary pressure to almost −48.3°C, the point at which ice crystals finally form spontaneously without any seed. The state is unstable. Drop in a crystal or jolt the container and ice spreads through the liquid almost instantly. Cool water fast enough, around a million degrees per second, and it skips crystals entirely and becomes a glass, a disordered solid.
Supercooled water is common in nature. Clouds are full of supercooled droplets, and when an aircraft flies through them the droplets can freeze onto its wings and instruments on impact, which is why planes need ice protection. Freezing rain forms the same way. Some fish, insects and reptiles survive cold by supercooling their body fluids, clearing out anything that might seed ice. Many northern plants use supercooling to survive down to about −40°C.
The reverse trick is much harder. A solid almost always melts at the same temperature, which is why scientists measure melting points rather than freezing points. Supercooling also has a playful commercial side: freezers can chill drinks into a supercooled state so they turn to slush when opened, and Coca-Cola once trialled vending machines built to do exactly that.
Source: Wikipedia — Supercooling · Text summarised from Wikipedia (CC BY-SA 4.0)
