Freezing is a
phase transition where a
liquid turns into a
solid when its
temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid or the liquid content of a substance, usually due to
cooling.
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).
Crystallization
Most liquids freeze by
crystallization, formation of
crystalline solid
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
from the uniform liquid. This is a first-order thermodynamic
phase transition, which means that as long as solid and liquid coexist, the temperature of the whole system remains very nearly equal to the
melting point due to the slow removal of heat when in contact with air, which is a poor heat conductor. Because of the
latent heat of fusion
In thermodynamics, the enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state fro ...
, the freezing is greatly slowed and the temperature will not drop anymore once the freezing starts but will continue dropping once it finishes.
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
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 before freezing. Under high pressure (2,000
atmospheres) water will supercool to as low as before freezing.
Exothermicity
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
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.
Expansion
''The size of substances increases or expands on being heated. This increase in the size of a body due to heating is called thermal expansion .. Thermal expansion takes place in all objects and in all states of matter. However different substances have different rates of expansion for the same rise in temperature.''
Freezing of living organisms
Many living organisms are able to tolerate prolonged periods of time at temperatures below the freezing point of water. Most living organisms accumulate
cryoprotectant
A cryoprotectant is a substance used to protect biological tissue from freezing damage (i.e. that due to ice formation). Arctic and Antarctic insects, fish and amphibians create cryoprotectants ( antifreeze compounds and antifreeze proteins) in ...
s 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
''Pseudomonas syringae'' is a rod-shaped, Gram-negative bacterium with polar flagella. As a plant pathogen, it can infect a wide range of species, and exists as over 50 different pathovars, all of which are available to researchers from inte ...
'', 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
''Carnobacterium pleistocenium'' is a recently discovered bacterium from the arctic part of Alaska. It was found in permafrost, seemingly frozen there for 32,000 years. Melting the ice, however, brought these extremophiles back to life. This i ...
'', as well as ''
Chryseobacterium greenlandensis
''Chryseobacterium aquaticum'' is a Gram-negative, non-spore-forming and non-motile bacteria
Non-motile bacteria are bacteria species that lack the ability and structures that would allow them to propel themselves, under their own power, thr ...
'' and ''
Herminiimonas glaciei
''Herminiimonas glaciei'' is a species of ultramicrobacterium in the family Oxalobacteraceae. These small gram-negative cells have a variable number of long flagella at the ends and sides of their rod-shaped bodies. With dimensions of 0.5–0. ...
'', 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
''Trichostrongylus'' species are nematodes (round worms), which are ubiquitous among herbivores worldwide, including cattle, sheep, donkeys, goats, deer, and rabbits.Garcia LS, editor. Diagnostic Medical Parasitology. 5 ed. Washington, DC: ASM Pr ...
'' and ''
Panagrolaimus davidi''. Many species of reptiles and amphibians survive freezing. See
cryobiology
Cryobiology is the branch of biology that studies the effects of low temperatures on living things within Earth's cryosphere or in science. The word cryobiology is derived from the Greek words κρῧος ryos "cold", βίος ios "life", and λ� ...
for a full discussion.
Human
gametes and 2-, 4- and 8-cell
embryos
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
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
Cryonics (from el, κρύος ''kryos'' meaning 'cold') is the low-temperature freezing (usually at ) and storage of human remains, with the speculative hope that resurrection may be possible in the future. Cryonics is regarded with skeptic ...
.
Food preservation
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 significantly extended shelf-life. Freezing preservation is generally regarded as superior to canning and dehydration with respect to retention in sensory attributes and nutritive attributes.
See also
*
Degree of frost
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Directional freezing
Directional freezing freezes from only one direction.
Directional freezing can freeze water, from only one direction or side of a container, into clear ice.
Directional freezing in a domestic freezer can be done by putting water in a insulated ...
*
Flash freezing
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Fractional freezing
*
Freezer
*
Freezing air temperature
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Frost
*
Micro-pulling-down
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Mpemba effect
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Phase diagram
Table
References
External links
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Video of an intermetallic compound solidifying/freezing
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Phase transitions
Atmospheric thermodynamics
Heating, ventilation, and air conditioning
Cryobiology
Articles containing video clips