Water’s freezing point, commonly known as 0°C (32°F), is a dynamic point influenced by chemical, physical, and biological factors. Freezing is the phase change from liquid to a solid crystalline structure, requiring water molecules to slow down and arrange themselves into an orderly lattice. Several mechanisms can interfere with this molecular ordering, allowing water to remain liquid even below its standard freezing point. These strategies range from simple chemistry to complex biological adaptations.
How Dissolved Substances Prevent Freezing
The most common method for preventing water from freezing involves adding dissolved substances, a concept explained by freezing point depression. This colligative property means the freezing point of a solvent, like water, is lowered when a solute is introduced. The depression is determined primarily by the concentration, or the number of dissolved particles, rather than the solute’s chemical identity.
When water cools, its molecules attempt to organize into the rigid, hexagonal structure characteristic of ice. Solute particles physically disrupt this orderly arrangement, interfering with the formation of the crystal lattice.
To overcome this disruption, the temperature must be lowered further, slowing the water molecules enough to bypass the solute interference. This principle is used in road salt (sodium chloride or calcium chloride) and in car engine coolant. A concentrated solution of calcium chloride, for instance, can lower the freezing point of water to approximately -30°C.
The Mechanism of Supercooling
Supercooling is a state where water is cooled below 0°C but remains liquid, demonstrating a kinetic barrier to freezing. Freezing requires a nucleation site—a starting point where the first ice crystal can form. In natural water, these sites are typically impurities, dust particles, or surface imperfections.
If water is highly purified and kept still, it can be cooled significantly below 0°C because it lacks the necessary nuclei. In the absence of an external nucleus, the water must rely on homogeneous nucleation, a much rarer event where molecules spontaneously align. Extremely pure water can be supercooled down to nearly -48.3°C before spontaneous freezing occurs.
This supercooled state is metastable, persisting only until a disturbance or the introduction of an ice crystal provides the required nucleation site. A sudden shock or the introduction of ice instantly triggers rapid crystallization. The process releases latent heat, which briefly raises the temperature of the newly formed ice-water mixture toward the standard freezing point.
Physical Conditions That Inhibit Ice Formation
External physical factors, primarily pressure and physical confinement, influence the phase transition of water. Unlike most substances, water is less dense as a solid, meaning ice takes up more volume than liquid water. This unique characteristic results in an inverse relationship between pressure and freezing point across a specific range.
Increasing pressure favors the liquid state because it resists the volume expansion required for freezing. Applying pressure slightly lowers the temperature at which water freezes. This effect is relatively minor, however, requiring hundreds of atmospheres of pressure to achieve a significant change.
A related factor is physical confinement, such as water held within very small pores or nano-spaces. When water is confined, the container’s geometry can kinetically inhibit the formation of the large, regular ice crystal lattice. This constraint prohibits the expansion needed for standard ice formation, effectively lowering the freezing point or leading to different, high-pressure ice polymorphs.
Biological Antifreeze Strategies
Living organisms, including fish, insects, and plants, have evolved sophisticated biological strategies to prevent freezing. They produce specialized molecules known as Antifreeze Proteins (AFPs) or Thermal Hysteresis Proteins. These proteins work through a non-colligative mechanism, meaning their effect is not simply due to particle concentration.
The function of AFPs is to physically bind to the surface of nascent ice crystals forming in the organism’s body fluids. By coating these tiny crystals, the proteins prevent them from growing larger and causing cellular damage. This action is described as an adsorption-inhibition mechanism.
This binding creates a gap between the melting point and the non-equilibrium freezing point, known as thermal hysteresis. Fish AFPs, for example, can create a thermal hysteresis of up to about -3.5°C. This kinetically inhibits ice growth, allowing organisms to survive in supercooled conditions without the ice crystals expanding to a fatal size.