Oxygen Is Actually Colorless but Turns Blue as a Liquid

The familiar, invisible gas we breathe undergoes a remarkable transformation when subjected to extreme cold, condensing into a liquid with a distinct pale blue hue. This color emerges because the oxygen molecules, now in close proximity, interact with light in a unique way. They absorb light from the red and yellow parts of the spectrum, which allows the blue wavelengths to pass through to our eyes. This phenomenon is a direct result of the quantum mechanical properties of oxygen's electrons. While it involves the interaction of light with oxygen, this process of absorption is different from the light-scattering effect that gives our atmosphere its blue appearance.

For a long time, scientists considered oxygen a "permanent gas," one that resisted all attempts at liquefaction. It wasn't until the 1870s that the first droplets were successfully produced. In 1877, Swiss physicist Raoul Pictet and French physicist Louis Paul Cailletet independently developed methods to liquefy oxygen. However, it was Polish scientists Zygmunt Wróblewski and Karol Olszewski who first produced a stable, measurable quantity of liquid oxygen in 1883 (Review).

Beyond its striking color, liquid oxygen possesses other intriguing qualities. It is strongly paramagnetic, meaning it is attracted to magnetic fields and can even be suspended between the poles of a powerful magnet. This property is caused by the same unpaired electrons responsible for its blue color. Furthermore, liquid oxygen is a potent oxidizing agent, capable of causing materials to burn with incredible energy. This characteristic has made it a crucial component in rocketry since the earliest liquid-fueled rockets, serving as the oxidizer that allows fuel to combust and generate immense thrust.