Oxygen, a chemical element represented by the diatomic molecule \(\text{O}_2\), is widely recognized as the gas that sustains life on Earth. While most people think of oxygen as completely invisible, which is true in its common low-density gaseous form, this perception of colorlessness depends on the element’s physical state. When oxygen is subjected to extreme cold or high pressure, its molecules pack together more tightly. This causes the element to display a surprising range of visible colors as it transitions from a gas to a liquid and through multiple solid phases.
Oxygen in its Gaseous State
In the gaseous state at standard temperature and pressure, oxygen appears transparent and colorless to the human eye. This is because individual \(\text{O}_2\) molecules do not absorb or scatter light within the visible spectrum effectively enough for us to perceive a hue. The energy transitions within a single oxygen molecule that cause light absorption primarily occur in the ultraviolet region of the electromagnetic spectrum. \(\text{O}_2\) molecules strongly absorb high-energy ultraviolet radiation, but this process has no effect on the visible light that passes through.
The extremely low density of the gas phase also contributes to the lack of color. In this state, the oxygen molecules are too far apart for their electronic states to be coupled efficiently. The blue appearance of the sky is sometimes mistakenly attributed to the color of gaseous oxygen itself. This atmospheric phenomenon is actually caused by Rayleigh scattering, where light is scattered by all the gas molecules, predominantly nitrogen and oxygen. This scattering effect, where shorter blue wavelengths are scattered more efficiently, is not the inherent color of the gas.
The Distinct Color of Liquid Oxygen
When oxygen gas is cooled below its boiling point of \(-183^\circ\text{C}\), it condenses into a clear, pale cyan liquid known as Liquid Oxygen (\(\text{LOX}\)). The appearance of this pale blue color is directly related to the massive increase in density from the gaseous to the liquid state. Liquid oxygen is approximately 860 times denser than its gaseous counterpart. The pale blue color results from a phenomenon known as a two-molecule/one-photon transition. In this process, a single photon of visible light in the red region of the spectrum possesses enough energy to simultaneously excite two adjacent \(\text{O}_2\) molecules. When the two molecules absorb this red light, the complementary color—blue—is transmitted and reflected, giving \(\text{LOX}\) its characteristic hue.
Color Changes in Solid Oxygen Phases
Further cooling liquid oxygen below its freezing point of \(-218.79^\circ\text{C}\) at atmospheric pressure causes it to solidify. At standard pressure, the initial solid oxygen phases (\(\alpha, \beta, \gamma\)) maintain a light sky-blue color, continuing the red light absorption mechanism seen in the liquid state. However, the physics of solid oxygen reveals a remarkable palette of colors under increasing pressure. When solid oxygen is subjected to immense pressure, the molecules are forced into new crystalline structures with altered electronic properties.
Pressure-Induced Color Changes
At room temperature and a pressure of about \(9\text{ GPa}\) (gigapascals), solid oxygen transitions into the \(\delta\)-phase, which is orange. Increasing the pressure beyond \(10\text{ GPa}\) leads to the \(\epsilon\)-phase, a dark-red to nearly black solid. In this phase, the \(\text{O}_2\) molecules aggregate into stable \(\text{O}_8\) clusters, or octaoxygen, which significantly changes how the material interacts with light. The color changes again at approximately \(96\text{ GPa}\), where the oxygen molecules re-structure into the \(\zeta\)-phase, transforming into a metallic black solid.