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physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistica ...
, supersaturation occurs with a
solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solutio ...
when the concentration of a solute exceeds the
concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', ''number concentration'', ...
specified by the value of
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...
at equilibrium. Most commonly the term is applied to a solution of a
solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
in a
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...
. A supersaturated solution is in a metastable state; it may be brought to equilibrium by forcing the excess of solute to separate from the solution. The term can also be applied to a mixture of gases.


History

Early studies of the phenomenon were conducted with
sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 mil ...
, also known as Glauber's Salt because, unusually, the solubility of this salt in water may decrease with increasing temperature. Early studies have been summarised by Tomlinson. It was shown that the crystallization of a supersaturated solution does not simply come from its agitation, (the previous belief) but from solid matter entering and acting as a "starting" site for crystals to form, now called "seeds". Expanding upon this, Gay-Lussac brought attention to the
kinematics Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a fiel ...
of salt ions and the characteristics of the container having an impact on the supersaturation state. He was also able to expand upon the number of salts with which a supersaturated solution can be obtained. Later Henri Löwel came to the conclusion that both nuclei of the solution and the walls of the container have a catalyzing effect on the solution that cause crystallization. Explaining and providing a model for this phenomenon has been a task taken on by more recent research. Désiré Gernez contributed to this research by discovering that nuclei must be of the same salt that is being crystallized in order to promote crystallization.


Occurrence and examples


Solid precipitate, liquid solvent

A solution of a chemical compound in a liquid will become supersaturated when the temperature of the
saturated solution In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...
is changed. In most cases solubility decreases with decreasing temperature; in such cases the excess of solute will rapidly separate from the solution as
crystals 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 ...
or an
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language, Gr ...
powder. In a few cases the opposite effect occurs. The example of sodium sulphate in water is well-known and this was why it was used in early studies of solubility. Recrystallization is a process used to purify chemical compounds. A mixture of the impure compound and solvent is heated until the compound has dissolved. If there is some solid impurity remaining it is removed by
filtration Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter ...
. When the temperature of the solution is subsequently lowered it briefly becomes supersaturated and then the compound crystallizes out until chemical equilibrium at the lower temperature is achieved. Impurities remain in the supernatant liquid. In some cases crystals do not form quickly and the solution remains supersaturated after cooling. This is because there is a thermodynamic barrier to the formation of a crystal in a liquid medium. Commonly this is overcome by adding a tiny crystal of the solute compound to the supersaturated solution, a process known as "seeding". Another process in common use is to rub a rod on the side of a glass vessel containing the solution to release microscopic glass particles which can act as nucleation centres. In industry,
centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...
is used to separate the crystals from the supernatant liquid. Some compounds and mixtures of compounds can form long-living supersaturated solutions.
Carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s are a class of such compounds; The thermodynamic barrier to formation of crystals is rather high because of extensive and irregular hydrogen bonding with the solvent, water. For example, although
sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refine ...
can be recrystallised easily, its hydrolysis product, known as "
invert sugar Inverted sugar syrup, also called invert syrup, invert sugar, simple syrup, sugar syrup, sugar water, bar syrup, syrup USP, or sucrose inversion, is a syrup mixture of the monosaccharides glucose and fructose, that is made by hydrolytic s ...
" or "golden syrup" is a mixture of
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
and
fructose Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorb ...
that exists as a viscous, supersaturated, liquid. Clear
honey Honey is a sweet and viscous substance made by several bees, the best-known of which are honey bees. Honey is made and stored to nourish bee colonies. Bees produce honey by gathering and then refining the sugary secretions of plants (primar ...
contains carbohydrates which may crystallize over a period of weeks. Supersaturation may be encountered when attempting to crystallize a protein.


Gaseous solute, liquid solvent

The solubility of a gas in a liquid increases with increasing gas pressure. When the external pressure is reduced, the excess gas comes out of solution. Fizzy drinks are made by subjecting the liquid to
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
, under pressure. In
champagne Champagne (, ) is a sparkling wine originated and produced in the Champagne wine region of France under the rules of the appellation, that demand specific vineyard practices, sourcing of grapes exclusively from designated places within it, ...
the CO2 is produced naturally in the final stage of
fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...
. When the bottle or can is opened some gas is released in the form of bubbles. Release of gas from the bloodstream can cause a deep-sea diver to suffer from
decompression sickness Decompression sickness (abbreviated DCS; also called divers' disease, the bends, aerobullosis, and caisson disease) is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompressio ...
(a.k.a. the bends) when returning to the surface. This can be fatal if the released gas enters the heart. Dissolved gases can be released during oil exploration when a strike is made. This occurs because the oil in oil-bearing rock is under considerable pressure from the over-lying rock, allowing the oil to be supersaturated with respect to dissolved gases.


Liquid formation from a mixture of gases

A
cloudburst A cloudburst is an extreme amount of precipitation in a short period of time, sometimes accompanied by hail and thunder, which is capable of creating flood conditions. Cloudbursts can quickly dump large amounts of water, e.g. 25 mm of prec ...
is an extreme form of production of liquid water from a supersaturated mixture of air and water vapour in the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
. Supersaturation in the vapour phase is related to the
surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...
of liquids through the
Kelvin equation The Kelvin equation describes the change in vapour pressure due to a curved liquid–vapor interface, such as the surface of a droplet. The vapor pressure at a convex curved surface is higher than that at a flat surface. The Kelvin equation is de ...
, the Gibbs–Thomson effect and the Poynting effect. The International Association for the Properties of Water and Steam (
IAPWS The International Association for the Properties of Water and Steam (IAPWS) is an international non-profit association of national organizations concerned with the properties of water and steam, particularly thermophysical properties and other aspe ...
) provides a special equation for the
Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature an ...
in the metastable-vapor region of water in its ''Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam''. All thermodynamic properties for the metastable-vapor region of water can be derived from this equation by means of the appropriate relations of thermodynamic properties to the Gibbs free energy.


Measurement

When measuring the concentration of a solute in a supersaturated gaseous or liquid mixture it is obvious that the pressure inside the
cuvette A cuvette (French: cuvette = "little vessel") is a small tube-like container with straight sides and a circular or square cross section. It is sealed at one end, and made of a clear, transparent material such as plastic, glass, or fused quartz. ...
may be greater than the ambient pressure. When this is so a specialized cuvette must be used. The choice of
analytical technique Analytical technique is a method used to determine a chemical or physical property of a chemical substance, chemical element, or mixture. There is a wide variety of techniques used for analysis, from simple weighing to advanced techniques using high ...
to use will depend on the characteristics of the analyte.


Applications

The characteristics of supersaturation have practical applications in terms of
pharmaceuticals A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy ( pharmacotherapy) is an important part of the medical field and re ...
. By creating a supersaturated solution of a certain drug, it can be ingested in liquid form. The drug can be made driven into a supersaturated state through any normal mechanism and then prevented from precipitating out by adding precipitation inhibitors. Drugs in this state are referred to as "supersaturating drug delivery services," or "SDDS." Oral consumption of a drug in this form is simple and allows for the measurement of very precise dosages. Primarily, it provides a means for drugs with very low solubility to be made into aqueous solutions. In addition, some drugs can undergo supersaturation inside the body despite being ingested in a crystalline form. This phenomenon is known as
in vivo supersaturation In vivo supersaturation is the behavior of orally administered compounds that undergo supersaturation In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the ...
. The identification of supersaturated solutions can be used as a tool for marine ecologists to study the activity of organisms and populations. Photosynthetic organisms release O2 gas into the water. Thus, an area of the ocean supersaturated with O2 gas can likely determined to be rich with photosynthetic activity. Though some O2 will naturally be found in the ocean due to simple physical chemical properties, upwards of 70% of all oxygen gas found in supersaturated regions can be attributed to photosynthetic activity. Supersaturation in vapor phase is usually present in the expansion process through steam
nozzle A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe. A nozzle is often a pipe or tube of varying cross sectional area, ...
s that operate with
superheated steam Superheated steam is steam at a temperature higher than its vaporization point at the absolute pressure where the temperature is measured. Superheated steam can therefore cool (lose internal energy) by some amount, resulting in a lowering of ...
at the inlet, which transitions to saturated state at the outlet. Supersaturation thus becomes an important factor to be taken into account in the design of
steam turbine A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam tu ...
s, as this results in an actual mass flow of steam through the nozzle being about 1 to 3% greater than the theoretically calculated value that would be expected if the expanding steam underwent a reversible adiabatic process through equilibrium states. In these cases supersaturation occurs due to the fact that the expansion process develops so rapidly and in such a short time, that the expanding vapor cannot reach its equilibrium state in the process, behaving as if it were superheated. Hence the determination of the expansion ratio, relevant to the calculation of the mass flow through the nozzle, must be done using an adiabatic index of approximately 1.3, like that of the superheated steam, instead of 1.135, which is the value that should have to be used for a quasi-static adiabatic expansion in the saturated region. The study of supersaturation is also relevant to atmospheric studies. Since the 1940s, the presence of supersaturation in the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
has been known. When water is supersaturated in the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. Fro ...
, the formation of ice lattices is frequently observed. In a state of saturation, the water particles will not form ice under tropospheric conditions. It is not enough for molecules of water to form an ice lattice at saturation pressures; they require a surface to condense on to or conglomerations of liquid water molecules of water to freeze. For these reasons, relative humidities over ice in the atmosphere can be found above 100%, meaning supersaturation has occurred. Supersaturation of water is actually very common in the upper troposphere, occurring between 20% and 40% of the time. This can be determined using satellite data from the Atmospheric Infrared Sounder.


References

{{Reflist Thermodynamics Atmospheric thermodynamics Underwater diving physics