Liquid nitrogen (\(\text{LN}_2\)) is simply nitrogen gas cooled down to a liquid state. It is classified as a cryogen, a substance that exists as a liquid at extremely low temperatures, typically defined as having a boiling point below \(-150^\circ\text{C}\) or \(-238^\circ\text{F}\). This colorless and odorless liquid is one of the most widely used coolants in modern science and industry due to its accessibility and intensely low temperature.
The Boiling Point of Liquid Nitrogen
The boiling point of liquid nitrogen, the temperature at which it rapidly converts from a liquid back into a gas, is approximately \(-196^\circ\text{C}\) (or \(-320.8^\circ\text{F}\)). In the scientific Kelvin scale, the boiling point is 77 K.
The liquid state is maintained within a very narrow window, as nitrogen’s freezing point is only slightly lower at about \(-210^\circ\text{C}\) (63 K). When liquid nitrogen encounters anything warmer than \(-196^\circ\text{C}\), it immediately boils, forming a protective layer of gas bubbles around the object, an effect known as the Leidenfrost effect. This rapid phase change drives its utility as a potent coolant.
How Extreme Cold is Utilized
Liquid nitrogen is leveraged across several fields where conventional freezing is insufficient. In medical and biological fields, \(\text{LN}_2\) is the standard for cryopreservation, storing biological materials for extended periods. This process halts all biological activity, preserving delicate items like sperm, egg cells, embryos, and tissue samples without degradation.
The ultra-fast freezing speed is employed in the food industry, a process often called Individual Quick Freezing (IQF). Rapid freezing minimizes the formation of large ice crystals inside food cells, which cause damage during slower freezing methods. This technique preserves the original texture, flavor, and nutritional structure of items upon thawing.
Industrially, the extreme cold is used for shrink-fitting. A metal part is cooled and temporarily shrunk to allow insertion into another component; as it warms, it expands to create a tight, permanent interference fit. \(\text{LN}_2\) is also used in electronics and manufacturing to cool vacuum pumps, creating the ultra-pure vacuum conditions necessary for producing sensitive components.
Essential Safety Considerations
Working with liquid nitrogen requires strict safety protocols due to two primary hazards: thermal injury and atmospheric displacement. Direct contact with the liquid or its frigid vapor causes immediate and severe tissue damage, resulting in cryogenic burns or frostbite. Specialized, loose-fitting insulated gloves and a face shield are required, as even a brief splash can freeze human tissue rapidly.
The second danger is asphyxiation, particularly in poorly ventilated or confined spaces. When one liter of liquid nitrogen boils, it expands to nearly 700 times its volume as a gas. This large volume of odorless and colorless nitrogen gas quickly displaces the oxygen in the surrounding air.
Because nitrogen offers no warning signs, a person can lose consciousness quickly if the oxygen concentration drops below the safe threshold of 19.5%. Pressure buildup is also a hazard if the liquid is stored in a sealed container without proper venting. As the liquid warms and converts back to gas, the pressure can increase dramatically, potentially causing the container to rupture or explode.