Oxygen, which makes up about 21% of the Earth’s atmosphere, exists as a gas at standard room temperature. It can be condensed into a liquid by cooling it to extremely low temperatures. The boiling point of oxygen, the temperature at which it transitions between liquid and gas states, is approximately 90.19 Kelvin at standard atmospheric pressure. This temperature translates to about -183 degrees Celsius or -297 degrees Fahrenheit, reflecting the weak attractive forces between oxygen molecules.
The Boiling Point in Context
The precise boiling point of 90.19 K places oxygen firmly in the range of cryogenic temperatures. In the Celsius scale, this corresponds to -182.96 °C, and the Fahrenheit equivalent is -297.33 °F. These temperatures are far below anything naturally experienced on Earth’s surface.
The scientific definition of a boiling point is the temperature at which the vapor pressure of a liquid equals the surrounding atmospheric pressure. For oxygen, this phase change occurs at the specified temperature only when the pressure is at one standard atmosphere. This temperature is significantly higher than that of nitrogen, the most abundant gas in the atmosphere, which boils at about -196 °C (77 K). Oxygen has a higher boiling point than nitrogen because its greater molecular mass leads to stronger London dispersion forces, requiring more energy to break apart.
Unique Properties of Liquid Oxygen
Once cooled below its boiling point, oxygen transforms into a liquid state, often abbreviated as LOX. This cryogenic liquid is visually a clear, pale cyan liquid, exhibiting a faint blue color. It is also slightly denser than liquid water, with a density of 1.141 kilograms per liter.
Liquid oxygen is paramagnetic, meaning it is attracted to a magnetic field. This attraction is a direct result of the oxygen molecule’s electronic structure, which contains two unpaired electrons. When LOX is poured between the poles of a strong horseshoe magnet, it will be temporarily held in place by the magnetic field. Working with liquid oxygen requires specialized handling due to its extreme cold and powerful oxidizing properties. Organic materials, such as asphalt or coal, can burn rapidly and energetically if they become soaked in LOX.
Essential Applications of Liquid Oxygen
Cooling oxygen into a liquid state allows for its efficient storage and transport. Converting oxygen gas to liquid compresses its volume significantly, making it possible to store a large amount of the element in a relatively small container. This high density is particularly useful in the aerospace industry, where liquid oxygen (LOX) is used as the oxidizer in rocket propellant. Combining LOX with a fuel like liquid hydrogen generates the thrust necessary to propel spacecraft into orbit.
Liquid oxygen is also used extensively in medical and industrial settings. In hospitals, cryogenic storage tanks containing LOX provide a reliable, high-purity source for oxygen therapy systems and ventilators. A single liquid oxygen tank can replace numerous high-pressure gas cylinders, simplifying logistics for healthcare facilities. Industrially, oxygen is vaporized from its liquid form for use in processes like steel manufacturing, where it increases combustion efficiency, and in welding and cutting, where it creates high-temperature flames.