Liquid nitrogen

Liquid nitrogen is nitrogen in a liquid state at an extremely low temperature. It is a colorless liquid with a density of 0.807 g/ml at its boiling point (−195.79 °C (77 K; −320 °F)) and a dielectric constant of 1.43.[1] Nitrogen was first liquefied at the Jagiellonian University on 15 April 1883 by Polish physicists, Zygmunt Wróblewski and Karol Olszewski.[2] It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is often referred to by the abbreviation, LN2 or "LIN" or "LN" and has the UN number 1977. Liquid nitrogen is a diatomic liquid, which means that the diatomic character of the covalent N bonding in N2 gas is retained after liquefaction.[3]

Liquid nitrogen is a cryogenic fluid that can cause rapid freezing on contact with living tissue. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. The temperature is held constant at 77 K by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks. The development of pressurised super-insulated vacuum vessels has enabled liquefied nitrogen to be stored and transported over longer time periods with losses reduced to 2% per day or less.[4]

The temperature of liquid nitrogen can readily be reduced to its freezing point 63 K (−210 °C; −346 °F) by placing it in a vacuum chamber pumped by a vacuum pump.[5] Liquid nitrogen's efficiency as a coolant is limited by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect, known as the Leidenfrost effect, applies to any liquid in contact with an object significantly hotter than its boiling point. Faster cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen rather than liquid nitrogen alone.

Liquidnitrogen
Liquid nitrogen
A demonstration of liquid nitrogen at the Freeside maker space in Atlanta, Georgia during the Online News Association conference in 2013
Nitrogen ice cream 0020
Students preparing homemade ice cream with liquid nitrogen.

Uses

Liquid nitrogen is a compact and readily transported source of dry nitrogen gas, as it does not require pressurization. Further, its ability to maintain temperatures far below the freezing point of water makes it extremely useful in a wide range of applications, primarily as an open-cycle refrigerant, including:

Culinary use of liquid nitrogen

The culinary use of liquid nitrogen is mentioned in an 1890 recipe book titled Fancy Ices authored by Mrs. Agnes Marshall,[14] but has been employed in more recent times by restaurants in the preparation of frozen desserts, such as ice cream, which can be created within moments at the table because of the speed at which it cools food.[14] The rapidity of chilling also leads to the formation of smaller ice crystals, which provides the dessert with a smoother texture.[14] The technique is employed by chef Heston Blumenthal who has used it at his restaurant, The Fat Duck to create frozen dishes such as egg and bacon ice cream.[14][15] Liquid nitrogen has also become popular in the preparation of cocktails because it can be used to quickly chill glasses or freeze ingredients.[16] It is also added to drinks to create a smoky effect, which occurs as tiny droplets of the liquid nitrogen come into contact with the surrounding air, condensing the vapour that is naturally present.[16]

Safety

Liquid Nitrogen Tank
Filling a liquid nitrogen Dewar from a storage tank
CUHK Liquid Nitrogen
A liquid nitrogen storage tank. It is a permanent structure.

Because the liquid-to-gas expansion ratio of nitrogen is 1:694 at 20 °C (68 °F), a tremendous amount of force can be generated if liquid nitrogen is rapidly vaporized in an enclosed space. In an incident on January 12, 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were malfunctioning and later sealed. As a result of the subsequent pressure buildup, the tank failed catastrophically. The force of the explosion was sufficient to propel the tank through the ceiling immediately above it, shatter a reinforced concrete beam immediately below it, and blow the walls of the laboratory 0.1–0.2 m off their foundations.[17]

Because of its extremely low temperature, careless handling of liquid nitrogen and any objects cooled by it may result in cold burns. In that case, special gloves should be used while handling. However, a small splash or even pouring down skin will not burn immediately because of the Leidenfrost effect, the evaporating gas thermally insulates to some extent, like touching a hot element very briefly with a wet finger. If the liquid nitrogen manages to pool anywhere, it will burn severely.

As liquid nitrogen evaporates it reduces the oxygen concentration in the air and can act as an asphyxiant, especially in confined spaces. Nitrogen is odorless, colorless, and tasteless and may produce asphyxia without any sensation or prior warning.[18][19][20]

Oxygen sensors are sometimes used as a safety precaution when working with liquid nitrogen to alert workers of gas spills into a confined space.[21]

Vessels containing liquid nitrogen can condense oxygen from air. The liquid in such a vessel becomes increasingly enriched in oxygen (boiling point 90 K; −183 °C; −298 °F) as the nitrogen evaporates, and can cause violent oxidation of organic material.[22]

Ingestion of liquid nitrogen can cause severe internal damage, due to freezing of the tissues which come in contact with it and to the volume of gaseous nitrogen evolved as the liquid is warmed by body heat. In 1997, a physics student demonstrating the Leidenfrost effect by holding liquid nitrogen in his mouth accidentally swallowed the substance, resulting in near-fatal injuries. This was apparently the first case in medical literature of liquid nitrogen ingestion.[23] In 2012, a young woman in England had her stomach removed after ingesting a cocktail made with liquid nitrogen.[24]

Production

Liquid nitrogen is produced commercially from the cryogenic distillation of liquified air or from the liquefication of pure nitrogen derived from air using pressure swing adsorption. An air compressor is used to compress filtered air to high pressure; the high-pressure gas is cooled back to ambient temperature, and allowed to expand to a low pressure. The expanding air cools greatly (the Joule–Thomson effect), and oxygen, nitrogen, and argon are separated by further stages of expansion and distillation. Small-scale production of liquid nitrogen is easily achieved using this principle. Liquid nitrogen may be produced for direct sale, or as a byproduct of manufacture of liquid oxygen used for industrial processes such as steelmaking. Liquid-air plants producing on the order of tons per day of product started to be built in the 1930s but became very common after the Second World War; a large modern plant may produce 3000 tons/day of liquid air products.[25]

See also

References

  1. ^ Murphy, E. J.; Morgan, S. O. "The Dielectric Properties of Insulating Materials" (PDF). Retrieved October 2, 2012.
  2. ^ Tilden, William Augustus (2009). A Short History of the Progress of Scientific Chemistry in Our Own Times. BiblioBazaar, LLC. p. 249. ISBN 1-103-35842-1.
  3. ^ Henshaw, D. G.; Hurst, D. G.; Pope, N. K. (1953). "Structure of Liquid Nitrogen, Oxygen, and Argon by Neutron Diffraction". Physical Review. 92: 1229. Bibcode:1953PhRv...92.1229H. doi:10.1103/PhysRev.92.1229.
  4. ^ DATA BOOK for Cryogenic Gases and Equipment. aspenycap.org
  5. ^ Umrath, W. (1974). "Cooling bath for rapid freezing in electron microscopy". Journal of Microscopy. 101: 103–105. doi:10.1111/j.1365-2818.1974.tb03871.x.
  6. ^ Wainner, Scott; Richmond, Robert (2003). The Book of Overclocking: Tweak Your PC to Unleash Its Power. No Starch Press. p. 44. ISBN 1-886411-76-X.
  7. ^ Karam, Robert D. (1998). Satellite Thermal Control for System Engineers. AIAA. p. 89. ISBN 1-56347-276-7.
  8. ^ Liquid Nitrogen Ice Cream Recipe, March 7, 2006
  9. ^ Liquid nitrogen – how to dose effectively, June 19, 2012
  10. ^ Chart Dosers Dosing Products, June 19, 2012
  11. ^ Harrabin, Roger (2 October 2012). "Liquid air 'offers energy storage hope'". BBC.
  12. ^ Markham, Derek (October 3, 2012). "Frozen Air Batteries Could Store Wind Energy for Peak Demand". Treehugger. Discovery Communications.
  13. ^ Dyer, Ted G. (February 2010). "Freeze-branding cattle" (PDF).
  14. ^ a b c d "Who What Why: How dangerous is liquid nitrogen?". BBC News. BBC. 9 October 2012. Retrieved 9 October 2012.
  15. ^ Wallop, Harry (9 October 2012). "The dark side of liquid nitrogen cocktails". The Daily Telegraph. Telegraph Media Group. Retrieved 12 October 2012.
  16. ^ a b Gladwell, Amy (9 October 2012). "Teenager's stomach removed after drinking cocktail". Newsbeat. BBC. Retrieved 9 October 2012.
  17. ^ Mattox, Brent S. "Investigative Report on Chemistry 301A Cylinder Explosion" (PDF). Texas A&M University. Archived from the original (reprint) on 2008-10-31.
  18. ^ British Compressed Gases Association (2000) BCGA Code of Practice CP30. The Safe Use of Liquid nitrogen Dewars up to 50 litres. Archived 2007-07-18 at the Wayback Machine ISSN 0260-4809.
  19. ^ Confined Space Entry - Worker and Would-be Rescuer Asphyxiated, Valero Refinery Asphyxiation Incident Case Study.
  20. ^ Inquiry after man dies in chemical leak, BBC News, October 25, 1999.
  21. ^ Liquid Nitrogen – Code of practice for handling. United Kingdom: Birkbeck, University of London. 2007. Retrieved 2012-02-08.
  22. ^ Levey, Christopher G. "Liquid Nitrogen Safety". Thayer School of Engineering at Dartmouth.
  23. ^ "Student Gulps Into Medical Literature". Worcester Polytechnic Institute. 20 January 1999. Archived from the original on 22 February 2014. Retrieved 11 October 2014.
  24. ^ Liquid nitrogen cocktail leaves teen in hospital, BBC News, October 8, 2012.
  25. ^ Almqvist, Ebbe (2003) History of Industrial Gases, Springer, ISBN 0306472775 p. 163
Acrokeratoelastoidosis of Costa

Acrokeratoelastoidosis of Costa is a familial condition characterized by multiple keratotic papules on the dorsum of the hands and feet, palms, soles, in which electron microscopy shows rarified, abnormal elastic tissue.It was characterized in 1953.Treatments such as liquid nitrogen, salicylic acid, tretinoin, and prednisone have been tried, though with limited success.

Cooling bath

A cooling bath, in laboratory chemistry practice, is a liquid mixture which is used to maintain low temperatures, typically between 13 °C and −196 °C. These low temperatures are used to collect liquids after distillation, to remove solvents using a rotary evaporator, or to perform a chemical reaction below room temperature (see: kinetic control).

Cooling baths are generally one of two types: (a) a cold fluid (particularly liquid nitrogen, water, or even air) — but most commonly the term refers to (b) a mixture of 3 components: (1) a cooling agent (such as dry ice or water ice); (2) a liquid 'carrier' (such as liquid water, ethylene glycol, acetone, etc.), which transfers heat between the bath and the vessel; ; and (3) an additive to depress the melting-point of the solid/liquid system.

A familiar example of this is the use of an ice/rock-salt mixture to freeze ice cream. Adding salt lowers the freezing temperature of water, lowering the minimum temperature attainable with only ice.

Cryogenic storage dewar

A cryogenic storage dewar (named after James Dewar) is a specialised type of vacuum flask used for storing cryogens (such as liquid nitrogen or liquid helium), whose boiling points are much lower than room temperature. Cryogenic storage dewars may take several different forms including open buckets, flasks with loose-fitting stoppers and self-pressurising tanks. All dewars have walls constructed from two or more layers, with a high vacuum maintained between the layers. This provides very good thermal insulation between the interior and exterior of the dewar, which reduces the rate at which the contents boil away. Precautions are taken in the design of dewars to safely manage the gas which is released as the liquid slowly boils. The simplest dewars allow the gas to escape either through an open top or past a loose-fitting stopper to prevent the risk of explosion. More sophisticated dewars trap the gas above the liquid, and hold it at high pressure. This increases the boiling point of the liquid, allowing it to be stored for extended periods. Excessive vapour pressure is released automatically through safety valves. The method of decanting liquid from a dewar depends upon its design. Simple dewars may be tilted, to pour liquid from the neck. Self-pressurising designs use the gas pressure in the top of the dewar to force the liquid upward through a pipe leading to the neck.

Cryogenics

In physics, cryogenics is the production and behaviour of materials at very low temperatures. A person who studies elements that have been subjected to extremely cold temperatures is called a cryogenicist.

It is not well-defined at what point on the temperature scale refrigeration ends and cryogenics begins, but scientists assume a gas to be cryogenic if it can be liquefied at or below −150 °C (123 K; −238 °F). The U.S. National Institute of Standards and Technology has chosen to consider the field of cryogenics as that involving temperatures below −180 °C (93 K; −292 °F). This is a logical dividing line, since the normal boiling points of the so-called permanent gases (such as helium, hydrogen, neon, nitrogen, oxygen, and normal air) lie below −180 °C while the Freon refrigerants, hydrocarbons, and other common refrigerants have boiling points above −180 °C.Discovery of superconducting materials with critical temperatures significantly above the boiling point of liquid nitrogen has provided new interest in reliable, low cost methods of producing high temperature cryogenic refrigeration. The term "high temperature cryogenic" describes temperatures ranging from above the boiling point of liquid nitrogen, −195.79 °C (77.36 K; −320.42 °F), up to −50 °C (223 K; −58 °F), the generally defined upper limit of study referred to as cryogenics.Cryogenicists use the Kelvin or Rankine temperature scale, both of which measure from absolute zero, rather than more usual scales such as Celsius or Fahrenheit, with their zeroes at arbitrary temperatures.

Cryonics Institute

Cryonics Institute (CI) is an American not-for-profit corporation that provides cryonics services. CI preserves humans and pets in liquid nitrogen after death with hopes of restoring them when new technology will be developed in the future.

Cryopreservation

Cryo-preservation or cryo-conservation is a process where organelles, cells, tissues, extracellular matrix, organs, or any other biological constructs susceptible to damage caused by unregulated chemical kinetics are preserved by cooling to very low temperatures (typically −80 °C using solid carbon dioxide or −196 °C using liquid nitrogen). At low enough temperatures, any enzymatic or chemical activity which might cause damage to the biological material in question is effectively stopped. Cryopreservation methods seek to reach low temperatures without causing additional damage caused by the formation of ice crystals during freezing. Traditional cryopreservation has relied on coating the material to be frozen with a class of molecules termed cryoprotectants. New methods are constantly being investigated due to the inherent toxicity of many cryoprotectants. By default it should be considered that cryopreservation alters or compromises the structure and function of cells unless it is proven otherwise for a particular cell population. Cryoconservation of animal genetic resources is the process in which animal genetic material is collected and stored with the intention of conservation of the breed.

Cryosurgery

Cryosurgery is the use of extreme cold in surgery to destroy abnormal or diseased tissue; thus, it is the surgical application of cryoablation. The term comes from the Greek words cryo (κρύο) ("icy cold") and surgery (cheirourgiki – χειρουργική) meaning "hand work" or "handiwork".

Cryosurgery has been historically used to treat a number of diseases and disorders, especially a variety of benign and malignant skin conditions.

Frozen food

Freezing food preserves it from the time it is prepared to the time it is eaten. Since early times, farmers, fishermen, and trappers have preserved grains and produce in unheated buildings during the winter season. Freezing food slows down decomposition by turning residual moisture into ice, inhibiting the growth of most bacterial species. In the food commodity industry, there are two processes: mechanical and cryogenic (or flash freezing). The freezing kinetics is important to preserve the food quality and texture. Quicker freezing generates smaller ice crystals and maintains cellular structure. Cryogenic freezing is the quickest freezing technology available due to the ultra low liquid nitrogen temperature −196 °C (−320 °F).Preserving food in domestic kitchens during modern times is achieved using household freezers. Accepted advice to householders was to freeze food on the day of purchase. An initiative by a supermarket group in 2012 (backed by the UK's Waste & Resources Action Programme) promotes the freezing of food "as soon as possible up to the product's 'use by' date". The Food Standards Agency was reported as supporting the change, providing the food had been stored correctly up to that time.

Gene bank

Gene banks are a type of biorepository which preserve genetic material. For plants, this could be by in vitro storage, freezing cuttings from the plant, or stocking the seeds (e.g. in a seedbank). For animals, this is the freezing of sperm and eggs in zoological freezers until further need. With corals, fragments are taken which are stored in water tanks under controlled conditions. Plant genetic material in a 'gene bank' is preserved at -196° Celsius in Liquid Nitrogen as mature seed (dry) or tissue (meristems).

Accession is the common term given to an individual sample in a gene bank, such as a distinct species or variety.

In plants, it is possible to unfreeze the material and propagate it, however, in animals, a living female is required for artificial insemination. While it is often difficult to use frozen animal sperm and eggs, there are many examples of it being done successfully.

In an effort to conserve agricultural biodiversity, gene banks are used to store and conserve the plant genetic resources of major crop plants and their crop wild relatives. There are many gene banks all over the world, with the Svalbard Global Seed Vault being probably the most famous one.The database of the largest gene banks in the world can be queried via a common website, Genesys.

Ground freezing

Ground freezing is a construction technique used in circumstances where soil needs to be stabilized so it will not collapse next to excavations, or to prevent contaminants spilled into soil from being leached away.

Ground freezing has been used for at least one hundred years.

Pipes are run through the soil to be frozen, and then refrigerants are run through the pipes, freezing the soil.

Frozen soil can be as hard as concrete.

Soil contaminated with radioactive elements that leaked from Japan's Fukushima Daiichi nuclear power plant was contained through ground freezing.A project in Boston known as the Big Dig used ground freezing during some of its tunneling, to allow its wide tunnels to be built under or through soil that supported existing infrastructure that would have been difficult or expensive to support using more traditional excavation methods.Some ground freezing projects use common salt brine as the refrigerant, but other projects benefit from using more exotic refrigerants, like liquid nitrogen.In Northern Canada and Arctic Alaska, passive pipe systems are used that do not require any external power to keep the ground frozen. These systems use in-ground evaporators and above-ground radiators filled with liquid refrigerant. When ambient temperatures fall below ground temperatures, the liquid vapor starts condensing in the radiator, reducing the pressure in the system causing the liquid in the evaporator to boil and evaporate. This process results in heat transfer from the ground to the air and keeps the ground in a permanent frozen state.

Ice cream

Ice cream (derived from earlier iced cream or cream ice) is a sweetened frozen food typically eaten as a snack or dessert. It may be made from dairy milk or cream, or soy, cashew, coconut or almondmilk, and is flavored with a sweetener, either sugar or an alternative, and any spice, such as cocoa or vanilla. Colourings are usually added, in addition to stabilizers. The mixture is stirred to incorporate air spaces and cooled below the freezing point of water to prevent detectable ice crystals from forming. The result is a smooth, semi-solid foam that is solid at very low temperatures (below 2 °C or 35 °F). It becomes more malleable as its temperature increases.

The meaning of the name "ice cream" varies from one country to another. Terms such as "frozen custard," "frozen yogurt," "sorbet," "gelato," and others are used to distinguish different varieties and styles. In some countries, such as the United States, "ice cream" applies only to a specific variety, and most governments regulate the commercial use of the various terms according to the relative quantities of the main ingredients, notably the amount of cream. Products that do not meet the criteria to be called ice cream are sometimes labelled "frozen dessert" instead. In other countries, such as Italy and Argentina, one word is used for all variants. Analogues made from dairy alternatives, such as goat's or sheep's milk, or milk substitutes (e.g., soy milk or tofu), are available for those who are lactose intolerant, allergic to dairy protein, or vegan.

Ice cream may be served in dishes, for eating with a spoon, or licked from edible cones. Ice cream may be served with other desserts, such as apple pie, or as an ingredient in ice cream floats, sundaes, milkshakes, ice cream cakes and even baked items, such as Baked Alaska.

Isopentane

Isopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. It is also the least dense liquid at standard conditions for temperature and pressure.The normal boiling point is just a few degrees above room temperature and isopentane will readily boil and evaporate away on a warm day. Isopentane is commonly used in conjunction with liquid nitrogen to achieve a liquid bath temperature of −160 °C. Natural gas typically contains 1% or less isopentane.

Liquid nitrogen cocktail

A liquid nitrogen cocktail is any mixed drink whose preparation involves the use of liquid nitrogen. Popularized as a novelty because of the smoky, bubbling "cauldron effect" it produces, liquid nitrogen is controversial as a cocktail ingredient because it boils at −196 °C (77 K; −321 °F) and its consumption is thus potentially lethal. However, it is not a regulated substance in most countries and there is little control of its use.

Liquid nitrogen engine

A liquid nitrogen vehicle is powered by liquid nitrogen, which is stored in a tank. Traditional nitrogen engine designs work by heating the liquid nitrogen in a heat exchanger, extracting heat from the ambient air and using the resulting pressurized gas to operate a piston or rotary motor. Vehicles propelled by liquid nitrogen have been demonstrated, but are not used commercially. One such vehicle, Liquid Air was demonstrated in 1902.

Liquid nitrogen propulsion may also be incorporated in hybrid systems, e.g., battery electric propulsion and fuel tanks to recharge the batteries. This kind of system is called a hybrid liquid nitrogen-electric propulsion. Additionally, regenerative braking can also be used in conjunction with this system.

In June 2016 trials will begin in London, UK on supermarket J. Sainsbury's fleet of food delivery vehicles: using a Dearman nitrogen engine to provide power for the cooling of food cargo when the vehicle is stationary and the main engine is off. Currently delivery lorries mostly have second smaller diesel engines to power cooling when the main engine is off.

Liquid nitrogen wash

The Liquid Nitrogen Wash is mainly used for the production of ammonia synthesis gas within fertilizer production plants. It is usually the last purification step in the ammonia production process sequence upstream of the actual ammonia production.

Liquid oxygen

Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of elemental oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard. It was later mostly supplanted by gaseous oxidizers such as nitrogen tetroxide because of easier storage, as is the case with the American Titan II missile from the Cold War.

Nitrogen

Nitrogen is a chemical element with symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is generally accorded the credit because his work was published first. The name nitrogène was suggested by French chemist Jean-Antoine-Claude Chaptal in 1790, when it was found that nitrogen was present in nitric acid and nitrates. Antoine Lavoisier suggested instead the name azote, from the Greek ἀζωτικός "no life", as it is an asphyxiant gas; this name is instead used in many languages, such as French, Russian, Romanian and Turkish, and appears in the English names of some nitrogen compounds such as hydrazine, azides and azo compounds.

Nitrogen is the lightest member of group 15 of the periodic table, often called the pnictogens. The name comes from the Greek πνίγειν "to choke", directly referencing nitrogen's asphyxiating properties. It is a common element in the universe, estimated at about seventh in total abundance in the Milky Way and the Solar System. At standard temperature and pressure, two atoms of the element bind to form dinitrogen, a colourless and odorless diatomic gas with the formula N2. Dinitrogen forms about 78% of Earth's atmosphere, making it the most abundant uncombined element. Nitrogen occurs in all organisms, primarily in amino acids (and thus proteins), in the nucleic acids (DNA and RNA) and in the energy transfer molecule adenosine triphosphate. The human body contains about 3% nitrogen by mass, the fourth most abundant element in the body after oxygen, carbon, and hydrogen. The nitrogen cycle describes movement of the element from the air, into the biosphere and organic compounds, then back into the atmosphere.

Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The extremely strong triple bond in elemental nitrogen (N≡N), the second strongest bond in any diatomic molecule after carbon monoxide (CO), dominates nitrogen chemistry. This causes difficulty for both organisms and industry in converting N2 into useful compounds, but at the same time means that burning, exploding, or decomposing nitrogen compounds to form nitrogen gas releases large amounts of often useful energy. Synthetically produced ammonia and nitrates are key industrial fertilisers, and fertiliser nitrates are key pollutants in the eutrophication of water systems.

Apart from its use in fertilisers and energy-stores, nitrogen is a constituent of organic compounds as diverse as Kevlar used in high-strength fabric and cyanoacrylate used in superglue. Nitrogen is a constituent of every major pharmacological drug class, including antibiotics. Many drugs are mimics or prodrugs of natural nitrogen-containing signal molecules: for example, the organic nitrates nitroglycerin and nitroprusside control blood pressure by metabolizing into nitric oxide. Many notable nitrogen-containing drugs, such as the natural caffeine and morphine or the synthetic amphetamines, act on receptors of animal neurotransmitters.

Snow City

Snow City is Singapore's first indoor snow centre located within the Science Centre Singapore area beside the Omni-Theatre in Jurong East.

Verrucae palmares et plantares

Verrucae palmares et plantares is a cutaneous condition characterized by warts on the palms and soles.

They are very contagious, and spread easily. Dry ice or liquid Nitrogen can get rid of them.

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