For most substances, the melting and freezing points are the same temperature; however, certain substances possess differing solid-liquid transition temperatures. For example,agar displays ahysteresis in itsmelting point and freezing point. It melts at 85 °C (185 °F) and solidifies from 32 to 40 °C (90 to 104 °F).[3]
Most liquids freeze by crystallization, formation ofcrystalline solid from the uniform liquid. This is a first-order thermodynamicphase transition, which means that as long as solid and liquid coexist, the temperature of the whole system remains very nearly equal to themelting point due to the slow removal of heat when in contact with air, which is a poor heat conductor.[citation needed] Because of thelatent heat of fusion, the freezing is greatly slowed and the temperature will not drop anymore once the freezing starts but will continue dropping once it finishes.[citation needed]
Crystallization consists of two major events,nucleation andcrystal growth. "Nucleation" is the step wherein the molecules start to gather into clusters, on thenanometer scale, arranging in a defined and periodic manner that defines thecrystal structure. "Crystal growth" is the subsequent growth of the nuclei that succeed in achieving the critical cluster size.
Rapid formation of ice crystals in supercool water (home freezer experiment)
Crystallization of pure liquids usually begins at a lower temperature than themelting point, due to highactivation energy ofhomogeneous nucleation. The creation of a nucleus implies the formation of an interface at the boundaries of the new phase. Some energy is expended to form this interface, based on thesurface energy of each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, and nucleation does not proceed. Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei. In presence of irregularities on the surface of the containing vessel, solid or gaseous impurities, pre-formed solid crystals, or other nucleators,heterogeneous nucleation may occur, where some energy is released by the partial destruction of the previous interface, raising the supercooling point to be near or equal to the melting point. The melting point ofwater at 1 atmosphere of pressure is very close to 0 °C (32 °F; 273 K), and in the presence ofnucleating substances the freezing point of water is close to the melting point, but in the absence of nucleators water cansupercool to −40 °C (−40 °F; 233 K) before freezing.[4][5] Under high pressure (2,000atmospheres) water will supercool to as low as −70 °C (−94 °F; 203 K) before freezing.[6]
Freezing is almost always anexothermic process, meaning that as liquid changes into solid, heat and pressure are released. This is often seen as counter-intuitive, since the temperature of the material does not rise during freezing, except if the liquid weresupercooled. But this can be understood since heat must be continually removed from the freezing liquid or the freezing process will stop. The energy released upon freezing is alatent heat, and is known as theenthalpy of fusion and is exactly the same as the energy required tomelt the same amount of the solid.
Low-temperaturehelium is the only known exception to the general rule.[7]Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K.Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must beadded to these substances in order to freeze them.[8]
Certain materials, such asglass andglycerol, may harden without crystallizing; these are calledamorphous solids. Amorphous materials, as well as some polymers, do not have a freezing point, as there is no abrupt phase change at any specific temperature. Instead, there is a gradual change in theirviscoelastic properties over a range of temperatures. Such materials are characterized by a glass transition that occurs at aglass transition temperature, which may be roughly defined as the "knee" point of the material's density vs. temperature graph. Because vitrification is a non-equilibrium process, it does not qualify as freezing, which requires an equilibrium between the crystalline and liquid state.
Many living organisms are able to tolerate prolonged periods of time at temperatures below the freezing point of water. Most living organisms accumulatecryoprotectants such asanti-nucleating proteins, polyols, and glucose to protect themselves againstfrost damage by sharp ice crystals. Most plants, in particular, can safely reach temperatures of −4 °C to −12 °C. Certainbacteria, notablyPseudomonas syringae, produce specialized proteins that serve as potent ice nucleators, which they use to force ice formation on the surface of various fruits and plants at about −2 °C.[9] The freezing causes injuries in the epithelia and makes the nutrients in the underlying plant tissues available to the bacteria.[10]
Freezing is a common method offood preservation that slows both food decay and the growth ofmicro-organisms. Besides the effect of lower temperatures onreaction rates, freezing makes water less available forbacteria growth. Freezing is a widely used method of food preservation. Freezing generally preserves flavours, smell and nutritional content. Freezing became commercially viable.
^Zachariassen KE, Kristiansen E (December 2000). "Ice nucleation and antinucleation in nature".Cryobiology.41 (4):257–79.doi:10.1006/cryo.2000.2289.PMID11222024.
