Liquid nitrogen



Liquid nitrogen (liquid density at the triple point is 0.807 g/mL) is the liquid produced industrially in large quantities by fractional distillation of liquid air and is often referred to by the abbreviation, LN2. It is pure nitrogen, in a liquid state. Liquid nitrogen has the UN number 1977.

Liquid nitrogen boils at -196 C, and is a cryogenic fluid which is potentially capable of causing rapid frostbite on contact with living tissue. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. Here, the very low temperature is held constant at -196 &deg;C 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.

Liquid nitrogen can easily be converted to the solid by placing it in a vacuum chamber pumped by a rotary vacuum pump. Liquid nitrogen freezes at -210 C. Despite its reputation, liquid nitrogen's efficiency as a coolant is reduced by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect is known as the Leidenfrost effect and applies to any liquid in contact with an object significantly hotter than its boiling point. More rapid cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen, than into liquid nitrogen alone. That said, liquid nitrogen alone is sufficient for most applications.

Applications
Liquid nitrogen is a compact and readily transported source of nitrogen gas without 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:
 * in the study of cryogenics
 * as a source of very dry nitrogen gas
 * the immersion freezing and transportation of food products
 * the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples and materials
 * the cryonic preservation of humans and pets in the hope of future reanimation.
 * as a cooling supplement for overclocking a central processing unit, a graphics processing unit, or another type of computer hardware


 * as a method of freezing water pipes in order to work on them in situations where a tap is not available to block water flow to the work area.
 * in cryotherapy for removing unsightly or potentially malignant skin lesions such as warts and actinic keratosis.
 * in the process of promession, a way to dispose of the dead.

Safety
Since the liquid to gas expansion ratio of this substance is 1:694, a tremendous amount of force can be generated when liquid nitrogen boils off for whatever reasons. In a well-known accident in 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs. As a result, the tank failed catastrophically, and exploded. The force of the explosion was sufficient to propel the tank through the floor/ceiling immediately above it.