Everything about Freezing totally explained
» For freezing as a method of food preservation, see frozen food. For a TV series of the same name, see Freezing (TV series). For freezing in terms of computers, see hang (computing)
In
physics and
chemistry,
freezing is the process whereby a
liquid turns to a
solid when cold enough. The
freezing point is the
temperature at which this happens.
Melting, the process of turning a solid to a liquid, is almost the exact opposite of freezing. All known liquids undergo freezing when the temperature is lowered with the sole exception of
helium, which remains
fluid at
absolute zero and can only be solidified under pressure. For most substances, the melting and freezing points are the same temperature, however, certain substances possess differing solid-liquid transition temperatures. For example,
agar melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F); this process is known as
thermal hysteresis.
Crystallization
Most liquids freeze by
crystallization, formation of
crystalline solid from the uniform liquid. This is a first-order thermodynamic
phase transition, which means that as long as solid and liquid coexist, the equilibrium temperature of the system remains constant and equal to the
melting point. Crystallization consists of two major events,
nucleation and
crystal growth. Nucleation is the step where the molecules start to gather into clusters, on the
nanometer scale, arranging in a defined and
periodic manner that defines the
crystal structure. The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size.
Supercooling
In spite of the
second law of thermodynamics, crystallization of pure liquids usually begins at 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 isn't enough to create its surface, and nucleation doesn't proceed. Freezing doesn't 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, rising 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
super cool to −42 °C (−43.6 °F, 231 K) before freezing. Under high pressure (2,000
atmospheres) water will super cool to as low as −70°C (−94°F, 203 K) before freezing.
Vitrification
Certain materials, such as
glass or
glycerol, may harden without crystallizing; these are called
amorphous solids. Amorphous materials as well as some polymers don't have a true freezing point as there's no abrupt phase change at any specific temperature. Instead, there's a gradual change in their
viscoelastic properties over a range of temperatures. Such materials are characterized by a
glass transition temperature which may be roughly defined as the "knee" point of the material's density vs. temperature graph.
Freezing of biological fluids
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.
Food preservation
Freezing is a common method of
food preservation which 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
bacterial growth.
Further Information
Get more info on 'Freezing'.
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