Water can remain liquid even when cooled below its typical freezing point of 0°C (32°F). This phenomenon is known as supercooling, where water exists in a liquid state at temperatures where it should be solid ice. This behavior makes supercooled water a subject of scientific interest and practical application.
Understanding Supercooled Water
Supercooled water is liquid water that has been cooled below its freezing point without solidifying.
Normally, when water cools to 0°C, its molecules begin to arrange into the rigid, crystalline structure of ice. However, for this crystallization to occur, a starting point or “nucleation site” is needed. These sites can be impurities, dust particles, or even tiny air bubbles within the water.
If water is exceptionally pure, free of nucleation sites, and kept perfectly still, it can bypass freezing and remain liquid. This metastable state is unstable; its transformation into ice is only a matter of time or disturbance. Water can be supercooled to approximately -48.3°C (-55°F) before it spontaneously freezes due to homogeneous nucleation, where ice crystals form without external initiation.
The Science of Instant Freezing
When supercooled water is disturbed, it rapidly solidifies. This disturbance can be a slight vibration, an impurity, or a tiny ice crystal, providing a nucleation site for water molecules to align and form a crystalline structure.
Once nucleation begins, the freezing process spreads almost instantaneously throughout the supercooled liquid. This rapid solidification is an exothermic process, meaning it releases heat. As the water molecules transition from a less ordered liquid state to a more ordered solid state, they release latent heat of fusion, causing a slight warming of the surrounding environment as the ice forms.
Applications of Supercooled Water
The unique properties of supercooled water have led to various practical applications across different fields.
In cryopreservation, supercooling preserves biological materials like organs or tissues without the damaging effects of ice crystal formation. By maintaining cells in a supercooled liquid state, cellular damage caused by sharp ice crystals can be minimized, improving the viability of preserved biological samples.
Supercooled water is also used in energy storage and thermal management systems. Its ability to absorb and release latent heat during phase change makes it suitable for efficient cooling or heating, such as in refrigeration or electronic device temperature regulation. Research also explores its use in de-icing technologies for aircraft wings or roadways, where a sudden phase change could prevent ice accumulation.
Observing Supercooling Safely
Supercooled water occurs naturally in various atmospheric conditions. For example, it is present in clouds, where water droplets can remain liquid at temperatures well below freezing. When these supercooled droplets come into contact with a surface, they can instantly freeze, leading to phenomena like freezing rain or rime ice, which can pose hazards to infrastructure and transportation.
Observing supercooling can be done with simple materials, though home experiments require caution and adult supervision if children are involved. Using purified bottled water, such as distilled or demineralized water, and cooling it carefully in a freezer without disturbance can result in supercooled water. The key is to avoid agitation during the cooling process and when handling the bottle.