Freezing
Liquid turns to solid upon cooling below freezing point.
Freezing is the process where a liquid becomes a solid after its temperature drops below a specific point, known as the freezing point. For most materials, this temperature is identical to the melting point, but some substances behave differently. Agar, for instance, melts at 85 °C but only solidifies between 32 and 40 °C.
Most liquids freeze through crystallization, which involves molecules arranging into a regular, repeating pattern. This is a first-order thermodynamic phase transition: while both solid and liquid are present, the system's temperature stays close to the melting point because heat is removed slowly through air, a poor conductor. The latent heat of fusion slows the process significantly—temperature stops dropping once freezing begins and only falls again after it finishes. Crystallization has two main steps: nucleation, where molecules form tiny, organized clusters on a nanometer scale, and crystal growth, where these clusters expand once they reach a critical size.
Pure liquids often freeze below their melting point due to the high energy needed for homogeneous nucleation. Forming a nucleus requires creating a new interface between phases, which costs energy based on surface tension. If a nucleus is too small, the energy released by forming its volume isn't enough to create its surface, so nucleation fails. Freezing only starts when the temperature is low enough to stabilize nuclei. However, irregularities on container surfaces, impurities, or existing crystals can trigger heterogeneous nucleation, which lowers the energy barrier and raises the freezing point closer to the melting point. For water at 1 atmosphere, the melting point is near 0 °C, and with nucleators present, freezing occurs at roughly the same temperature. Without nucleators, water can supercool to −40 °C before freezing, and under 2,000 atmospheres of pressure, it can supercool to −70 °C.
Freezing is almost always exothermic, releasing heat and pressure as the liquid solidifies. This seems counterintuitive because the material's temperature doesn't rise during freezing (unless it was supercooled), but heat must be continuously removed for the process to continue. The energy released is the latent heat of fusion, equal to the energy needed to melt the same amount of solid. The only known exceptions are low-temperature helium isotopes: helium-3 has a negative enthalpy of fusion below 0.3 K, and helium-4 below 0.8 K, meaning heat must be added to freeze them at constant pressures.
Some materials, like glass and glycerol, harden without crystallizing, forming amorphous solids. These substances, along with certain polymers, lack a distinct freezing point because there is no abrupt phase change. Instead, their viscoelastic properties shift gradually over a temperature range, characterized by a glass transition temperature—the "knee" in a density-versus-temperature graph. Since vitrification is a non-equilibrium process, it does not count as freezing, which requires equilibrium between crystalline and liquid states.
Many living organisms can survive temperatures below water's freezing point. They accumulate cryoprotectants—such as anti-nucleating proteins, polyols, and glucose—to prevent damage from sharp ice crystals. Most plants safely withstand temperatures from −4 °C to −12 °C. Certain bacteria, like *Pseudomonas syringae*, produce potent ice-nucleating proteins that force ice formation on fruits and plants at about −2 °C, causing injuries that release nutrients for the bacteria.
Three bacterial species—*Carnobacterium pleistocenium*, *Chryseobacterium greenlandensis*, and *Herminiimonas glaciei*—have been revived after thousands of years frozen in ice. Many plants undergo a hardening process to survive below 0 °C for weeks or months. Among animals, the nematode *Haemonchus contortus* can survive 44 weeks frozen in liquid nitrogen, and other nematodes like *Trichostrongylus colubriformis* and *Panagrolaimus davidi* also tolerate freezing. Numerous reptiles and amphibians survive freezing as well. Human gametes and embryos at the 2-, 4-, and 8-cell stages can be frozen and remain viable for up to 10 years through cryopreservation. Experimental attempts to freeze whole human beings for later revival have also been undertaken.
- type
- phase transition
- common_example
- water
- exception
- Helium-3 and Helium-4 have negative enthalpy of fusion at low temperatures
Lore & Background
Freezing most often occurs by crystallization, a first-order thermodynamic phase transition. As solid and liquid coexist, the temperature remains near the melting point due to latent heat of fusion. Crystallization involves nucleation, where molecules form nanometer-scale clusters, and crystal growth. Pure liquids often supercool, beginning to freeze below the melting point due to the energy needed to form a new interface. Heterogeneous nucleation, aided by impurities or surface irregularities, raises the freezing point closer to the melting point. Water can supercool to −40 °C without nucleators, and to −70 °C under high pressure.
Reader's Guide
Freezing is almost always exothermic, releasing latent heat equal to the enthalpy of fusion. Low-temperature helium is the only known exception, requiring heat addition to freeze. Vitrification, as seen in glass and glycerol, is not true freezing because it lacks an equilibrium crystalline phase. Freezing is critical for food preservation, slowing decay and microbial growth while preserving flavor and nutrients. Many organisms survive freezing by accumulating cryoprotectants; some bacteria, like Pseudomonas syringae, use ice-nucleating proteins to induce freezing on plants. Certain bacteria have been revived after thousands of years frozen in ice. Human gametes and embryos can survive freezing for up to 10 years via cryopreservation.
Did You Know?
- Agar displays hysteresis: it melts at 85 °C but solidifies between 32 and 40 °C.
- Helium-3 and Helium-4 have negative enthalpy of fusion at very low temperatures, requiring heat addition to freeze.
- Three bacterial species—Carnobacterium pleistocenium, Chryseobacterium greenlandensis, and Herminiimonas glaciei—have been revived after thousands of years frozen in ice.
The Particle-Level Mechanics of Freezing
When a liquid undergoes freezing, the fundamental change occurs in how its constituent particles interact and move. In the liquid state, molecules remain relatively close to one another and intermolecular forces still play a significant role, yet the particles possess enough energy to slide past their neighbors, giving the substance a mobile, shape-shifting structure. As the temperature drops and the liquid crosses below its melting point—provided pressure exceeds the triple point—those intermolecular forces gain dominance. The particles lose the kinetic energy needed to maintain their relative mobility and become locked into fixed positions where they can only vibrate in place. This transition from a nearly incompressible, container-conforming fluid to a material with a definite shape and volume is what we call freezing. The resulting solid may be crystalline, with atoms or molecules arranged in a repeating, ordered lattice, or it may lack long-range order entirely, as in amorphous materials such as glass.
Freezing Among the Full Family of Phase Transitions
Freezing does not exist in isolation; it is one link in a chain of transformations connecting all the classical states of matter. A solid can melt into a liquid, and that liquid can freeze back into a solid. Alternatively, a solid can skip the liquid phase entirely and transition directly into a gas through sublimation, while a gas can deposit directly into a solid without ever passing through the liquid stage. The direction and feasibility of each transition depend critically on temperature and pressure. A liquid converts to a gas when heated to its boiling point at constant pressure, or when pressure is reduced at constant temperature. The highest temperature at which a liquid can exist is its critical temperature, beyond which the boundary between liquid and gas dissolves into a supercritical fluid. Freezing, then, is the specific reverse of melting, governed by the same thermodynamic boundaries, and it sits at the intersection of multiple possible pathways through the phase diagram of any given substance.
Water's Exception and the Multiplicity of Solid Phases
One of the most striking facts about freezing is that the resulting solid is not always denser than the liquid it came from. Water stands as the best-known exception to the general rule: for most substances, the liquid occupies more volume than the corresponding solid, but water reverses this relationship. Beyond this density quirk, the solid state itself is far more varied than a single frozen form. Ice, for example, is not one thing but fifteen distinct crystal structures, each stable at different combinations of temperature and pressure. This means that freezing a substance does not produce a single, universal solid; rather, the precise crystal phase that emerges depends on the thermodynamic path taken, and a single compound can yield multiple solid phases that are all genuinely the same state of matter.
Freezing Beyond the Classical: Amorphous Solids and Extreme Conditions
The familiar image of a liquid crystallizing into an ordered solid is only one outcome of the freezing process. Glasses and other amorphous solids lack long-range atomic order and are not true thermal equilibrium ground states; they represent a kind of arrested, non-equilibrium solidification. This places them in a category of nonclassical states of matter distinct from the classical solid-liquid-gas-plasma framework. At the other extreme, matter can be driven into entirely different ordered states under conditions far removed from everyday freezing. Bose–Einstein condensates and Fermionic condensates emerge at extreme cold, while neutron-degenerate matter appears under extreme density and quark–gluon plasma requires extremely high energy. Even within a conventional solid where atoms are locked in place, the intrinsic magnetic moments of those atoms can organize into distinct configurations such as ferromagnetism or antiferromagnetism, revealing that freezing into a solid is only the beginning of the structural and energetic complexity matter can exhibit.
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