Liquid nitrogen (liquid density at the triple point is 0.707 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 77 K (–196 °C/–321 °F), and is a cryogenic fluid which can cause rapid frostbite on contact with living tissue. It has a dielectric constant of 1.4.[1] 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 77K 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.[2] Liquid nitrogen freezes at 63 K (–210 °C/–346 °F). 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.

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