Liquid nitrogen (LN2) isnitrogen in aliquid state atlow temperature. Liquid nitrogen has aboiling point of about −196 °C (−321 °F; 77 K). It is produced industrially byfractional distillation ofliquid air. It is a colorless, mobileliquid whoseviscosity is about one-tenth that ofacetone (i.e. roughly one-thirtieth that ofwater atroom temperature). Liquid nitrogen is widely used as acoolant.
The diatomic character of the N2 molecule is retained afterliquefaction. The weakvan der Waals interaction between the N2 molecules results in little interatomic attraction. This is the cause of nitrogen's unusually lowboiling point.[1]
The temperature of liquid nitrogen can readily be reduced to its freezing point −210 °C (−346 °F; 63 K) by placing it in a vacuum chamber pumped by avacuum pump.[2] 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 an insulating layer of nitrogen gas bubbles. This effect, known as theLeidenfrost effect, occurs when any liquid comes in contact with a surface which is significantly hotter than its boiling point. Faster cooling may be obtained by plunging an object into a slush of liquid andsolid nitrogen rather than liquid nitrogen alone.[2]
As acryogenic fluid that rapidlyfreezes living tissue, its handling and storage requirethermal insulation. It can be stored and transported invacuum flasks, the temperature being held constant at 77 K by slow boiling of the liquid. 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 liquid nitrogen to be stored and transported over longer time periods with losses reduced to 2 percent per day or less.[3]
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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 thefreezing point of water,specific heat of 1040J⋅kg−1⋅K−1 andheat of vaporization of 200 kJ⋅kg−1 makes it extremely useful in a wide range of applications, primarily as an open-cyclerefrigerant, including:
The culinary use of liquid nitrogen is mentioned in an 1890 recipe book titledFancy Ices authored byAgnes Marshall.[13] It 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.[13] The rapidity of chilling also leads to the formation of smaller ice crystals, which provides the dessert with a smoother texture.[13] The technique is employed by chefHeston Blumenthal who has used it at his restaurant,The Fat Duck, to create frozen dishes such as egg and bacon ice cream.[13][14] Some bartenders use liquid nitrogen to quickly chill glasses, freeze ingredients, or add it as a featured ingredient inliquid nitrogen cocktails.[15] It creates a smoky effect in a drink, which occurs as tiny droplets of the liquid nitrogen come into contact with the surrounding air, condensing the vapour that is naturally present.[15]
Nitrogen was first liquefied at theJagiellonian University on 15 April 1883 by Polish physicistsZygmunt Wróblewski andKarol Olszewski.[16]
Because the liquid-to-gasexpansion ratio of nitrogen is 1:694 at 20 °C (68 °F), a tremendous amount of force can be generated if liquid nitrogen is vaporized in an enclosed space. In an incident on January 12, 2006 atTexas 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] In January 2021, a line carrying liquid nitrogen ruptured at a poultry processing plant in the U.S. state of Georgia, killing six people and injuring 11 others.[18]
Because of its extremely low temperature, careless handling of liquid nitrogen and any objects cooled by it may result incold 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 theLeidenfrost 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 theoxygen concentration in theair and can act as anasphyxiant, especially inconfined spaces. Nitrogen is odorless, colorless, and tasteless and may produceasphyxia without any sensation or prior warning.[19][20][21]
Oxygen sensors are sometimes used as a safety precaution when working with liquid nitrogen to alert workers of gas spills into a confined space.[22]
Vessels containing liquid nitrogen cancondense 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.[23]
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.[24] 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.[25] In 2012, a young woman in England had her stomach removed after ingesting a cocktail made with liquid nitrogen.[26]
Liquid nitrogen is produced commercially from the cryogenicdistillation ofliquified air or from the liquefaction of pure nitrogen derived from air usingpressure swing adsorption. Anair 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 (theJoule–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.[27] Liquid nitrogen may be produced for direct sale, or as a byproduct of manufacture of liquid oxygen used for industrial processes such assteelmaking. 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.[28]
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