Liquid nitrogen

Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, mobile liquid whose viscosity is about one tenth that of acetone. Liquid nitrogen is widely used as a coolant. The diatomic character of the N2 molecule is retained after liquefaction. The weak van der Waals interaction between the N2 molecules results in little interatomic attraction. This is the cause of nitrogen's unusually low boiling point. The temperature of liquid nitrogen can readily be reduced to its freezing point by placing it in a vacuum chamber pumped by a vacuum pump. 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 the Leidenfrost 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 and solid nitrogen rather than liquid nitrogen alone. As a cryogenic fluid that rapidly freezes living tissue, its handling and storage require thermal insulation. It can be stored and transported in vacuum 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% per day or less. Liquid nitrogen is a compact and readily transported source of dry nitrogen gas, as it does not require pressurization.