Liquid oxygen

Liquid oxygen, often called LOX, is a clear, pale cyan liquid form of dioxygen. It is a cryogenic substance, which means it exists at extremely low temperatures. This liquid is vital for both modern technology and human life. It serves as a powerful oxidizer for rocket engines. It also provides a concentrated source of oxygen for medical and military use.

To understand how we get this liquid, we must look at the air around us. Scientists use a process called fractional distillation in cryogenic air separation plants. This method separates the oxygen from the natural air. Liquid oxygen has a density of 1.141 g/cm³, making it slightly denser than liquid water. It has a freezing point of -218.8 °C (54.4 K) and a boiling point of -183 °C (90 K). Because of its high expansion ratio of 1:861, it expands greatly when it turns into a gas.

One of the most fascinating properties of liquid oxygen is its paramagnetism. This means the liquid is attracted to magnetic fields. If you pour it between the poles of a powerful horseshoe magnet, it will stay suspended there. This behavior led Gilbert N. Lewis to predict the existence of the tetraoxygen molecule (O4) in 1924. He proposed this to explain why the liquid defied Curie's law. While modern computer simulations show that stable O4 molecules do not exist, they show that O2 molecules form transient O4 units by associating in pairs.

Liquid oxygen is also a very powerful oxidizing agent. An oxidizer is a substance that enables or accelerates combustion. Because it is so reactive, organic materials will burn very rapidly and energetically when they touch it. Some materials, such as coal briquettes or carbon black, can even detonate unpredictably if soaked in it. This can be caused by flames, sparks, or even the impact of light blows. Even petrochemicals like asphalt can show this dangerous behavior.

History shows us the long struggle to master these cold substances. By 1845, Michael Faraday had liquefied many known gases. However, oxygen was one of six "permanent gases" that resisted his attempts. In 1877, Louis Paul Cailletet and Raoul Pictet succeeded in producing the first droplets of liquid air. Finally, in 1883, Polish professors Zygmunt Wróblewski and Karol Olszewski produced the first measurable quantity of liquid oxygen. These discoveries paved the way for modern aerospace engineering.

Liquid oxygen is the most common cryogenic liquid oxidizer used for spacecraft propulsion. It is usually paired with fuels like liquid hydrogen, kerosene, or methane. Robert H. Goddard used it in the first liquid-fueled rocket in 1926. During World War II, the V-2 missile used it under the names A-Stoff and Sauerstoff. In the 1950s, the Redstone, Atlas, and R-7 Semyorka rockets all relied on it. Later, the Apollo Saturn rockets and the Space Shuttle main engines used LOX to reach orbit.

Today, liquid oxygen remains essential for global space programs. Many active rockets use it, including the SpaceX Falcon 9 and Starship. NASA uses it for the Space Launch System. The European Space Agency uses it for the Ariane 6. Many Chinese rockets, such as the Long March family, also use it. Other nations like India, Japan, Korea, and Russia continue to use LOX in their rocket systems.

Beyond space, liquid oxygen has critical roles in daily life. It is classified as an industrial gas for medical and commercial use. In hospitals, it is used to help patients breathe. The U.S. Air Force uses it for high-altitude aircraft flights. In 1985, the USAF even began building its own oxygen-generation facilities at major bases. This ensures a steady supply of oxygen for both breathing and strategic mission needs.