Мржњење

Water dripping from a slab of ice and then freezing, forming icicles.

Мржњење или очвршћавање је физички процес у коме се материја из течног стања претвара у чврсто агрегатно стање. До ове појаве долази хлађењем течности до температуре мржњења. Топљење је обрнут процес у ком се чврсто тело претвара у течност. Све познате течности, са изузетком течног хелијума, могу се хлађењем превести у чврсто стање. За већину супстанци тачке топљења и мржњења су на истој температури, док се за мањи број њих ове температуре разликују. Ова појава се назива хистерезис. Рецимо, агар се топи на 85°C, а мрзне се на 31°C до 40°C.

Температура супстанце остаје непромењена све док се не изврши потпун фазни прелаз у чврсто стање. При мржњењу тело предаје топлотну енергију околини, иако се његова температура не мења. Ако се овај трансфер топлоте спречи мржњење се зауставља. Ова одведена топлота се назива латентна топлота. Сличан, али обрнут термодинамички процес се одвија приликом топљења.

Већина течности се мрзну кристализацијом. Она се састоји из фазе формирања нуклеуса и фазе његовог раста.

Неке материје, као што су стакло и глицерол, мрзну се без кристализације. Оне се називају аморфна чврста тела. Њихов прелаз у чврсто стање је постепен и одвија се у извесном температурном опсегу. Прелаз оваквих материјала у чврсто стање назива се витрификација.

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 a hysteresis in its melting point and freezing point. It melts at 85°C (185°F) and solidifies from 32°C to 40°C (89.6°F to 104°F).

Кристализација

Главни чланак: Кристализација

Crystallization consists of two major events, nucleation and crystal growth. "Nucleation" is the step wherein the molecules start to gather into clusters, on the nanometer scale, arranging in a defined and periodic manner that defines the crystal structure. "Crystal growth" is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. The thermodynamics of freezing and melting is a classical discipline within physical chemistry, which nowadays develops in conjunction with computer simulations.

Supercooling

Главни чланак: Supercooling

Rapid formation of ice crystals in supercool water (home freezer experiment)

In spite of the second law of thermodynamics, crystallization of pure liquids usually begins at a lower temperature than the melting point, due to high activation energy of homogeneous 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 the surface 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 of water at 1 atmosphere of pressure is very close to 0°C (32°F, 273.15 K), and in the presence of nucleating substances the freezing point of water is close to the melting point, but in the absence of nucleators water can supercool to −40 °C (−40 °F; 233 K) before freezing. Under high pressure (2,000 atmospheres) water will supercool to as low as −70 °C (−94 °F; 203 K) before freezing.

Exothermicity

Главни чланак: Enthalpy of fusion

Freezing is almost always an exothermic 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 were supercooled. 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 a latent heat, and is known as the enthalpy of fusion and is exactly the same as the energy required to melt the same amount of the solid.

Low-temperature helium is the only known exception to the general rule. 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 be added to these substances in order to freeze them.

Vitrification

Главни чланак: Glass transition

Certain materials, such as glass and glycerol, may harden without crystallizing; these are called amorphous 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 their viscoelastic properties over a range of temperatures. Such materials are characterized by a glass transition that occurs at a glass 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.

Freezing of living organisms

Главни чланак: Cryobiology

Many living organisms are able to tolerate prolonged periods of time at temperatures below the freezing point of water. Most living organisms accumulate cryoprotectants such as anti-nucleating proteins, polyols, and glucose to protect themselves against frost damage by sharp ice crystals. Most plants, in particular, can safely reach temperatures of −4 °C to −12 °C. Certain bacteria, notably Pseudomonas 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. The freezing causes injuries in the epithelia and makes the nutrients in the underlying plant tissues available to the bacteria.

Bacteria

Three species of bacteria, Carnobacterium pleistocenium, as well as Chryseobacterium greenlandensis and Herminiimonas glaciei, have reportedly been revived after surviving for thousands of years frozen in ice.

Plants

Many plants undergo a process called hardening, which allows them to survive temperatures below 0 °C for weeks to months.

Animals

The nematode Haemonchus contortus can survive 44 weeks frozen at liquid nitrogen temperatures. Other nematodes that survive at temperatures below 0 °C include Trichostrongylus colubriformis and Panagrolaimus davidi. Many species of reptiles and amphibians survive freezing. See cryobiology for a full discussion.

Human gametes and 2-, 4- and 8-cell embryos can survive freezing and are viable for up to 10 years, a process known as cryopreservation.

Experimental attempts to freeze human beings for later revival are known as cryonics.

Food preservation

Главни чланак: Frozen food

Freezing is a common method of food preservation that slows both food decay and the growth of micro-organisms. Besides the effect of lower temperatures on reaction rates, freezing makes water less available for bacteria growth. freezing is one of the oldest and most widely used method of food preservation as far back as 1842, freezing has been immensely used in an ice and salt brine. In freezing, flavours, smell and nutritional content most generally remain unchanged. Freezing became commercially applicable after the advent (introduction) of mechanical refrigeration. Freezing has been successfully employed for long term preservation of many foods providing a significant extended shelf-life. Freezing preservation is generally regarded as superior to canning and dehydration with respect to retention in sensory attributes and nutritive attributes.

Види још

• Суперхлађење
• Топљење

Литература

• A. Mersmann, Crystallization Technology Handbook (2001) CRC; 2nd ed. ISBN 0-8247-0528-9
• Tine Arkenbout-de Vroome, Melt Crystallization Technology (1995) CRC ISBN 1-56676-181-6
• "Small Molecule Crystallization" (PDF) at Illinois Institute of Technology website
• Glynn P.D. and Reardon E.J. (1990) "Solid-solution aqueous-solution equilibria: thermodynamic theory and representation". Amer. J. Sci. 290, 164–201.
• Geankoplis, C.J. (2003) "Transport Processes and Separation Process Principles". 4th Ed. Prentice-Hall Inc.
• S.J. Jancic, P.A.M. Grootscholten: “Industrial Crystallization”, Textbook, Delft University Press and Reidel Publishing Company, Delft, The Netherlands, 1984.
• Giovambattista, N.; Angell, C. A.; Sciortino, F.; Stanley, H. E. (јул 2004). „Glass-Transition Temperature of Water: A Simulation Study” (PDF). Physical Review Letters. 93 (4): 047801. Bibcode:2004PhRvL..93d7801G. PMID 15323794. S2CID 8311857. arXiv:cond-mat/0403133 . doi:10.1103/PhysRevLett.93.047801. CS1 одржавање: Формат датума (веза)
• Rogerson, M. A.; Cardoso, S. S. S. (април 2004). „Solidification in heat packs: III. Metallic trigger”. AIChE Journal. 49 (2): 522—529. doi:10.1002/aic.690490222. Архивирано из оригинала 2012-12-09. г. CS1 одржавање: Формат датума (веза)
• O. Gomes, Gabriel; Stanley, H. Eugene; Souza, Mariano de (2019-08-19). „Enhanced Grüneisen Parameter in Supercooled Water”. Scientific Reports (на језику: енглески). 9 (1): 12006. Bibcode:2019NatSR...912006O. ISSN 2045-2322. PMC 6700159 . PMID 31427698. arXiv:1808.00536 . doi:10.1038/s41598-019-48353-4. 
• Rathz, Tom. „Undercooling”. NASA. Архивирано из оригинала 2009-12-02. г. Приступљено 2010-01-12. 
• Moore, Emily; Valeria Molinero (24. 11. 2011). „structural transformation in supercooled water controls the crystallization rate of ice”. Nature. 479 (7374): 506—508. Bibcode:2011Natur.479..506M. PMID 22113691. S2CID 1784703. arXiv:1107.1622 . doi:10.1038/nature10586. CS1 одржавање: Формат датума (веза)
• Debenedetti, P. G.; Stanley, H. E. (2003). „Supercooled and Glassy Water” (PDF). Physics Today. 56 (6): 40–46 [p. 42]. Bibcode:2003PhT....56f..40D. doi:10.1063/1.1595053. 
• Angell, C. Austen (2008). „Insights into Phases of Liquid Water from Study of Its Unusual Glass-Forming Properties”. Science. 319 (5863): 582—587. PMID 18239117. S2CID 9860383. doi:10.1126/science.1131939. 
• Ramsay, J. A. (1949). „A new method of freezing-point determination for small quantities” (PDF). J. Exp. Biol. 26 (1): 57—64. PMID 15406812. doi:10.1242/jeb.26.1.57. 

Спољашње везе

• Video of an intermetallic compound solidifying/freezing