Oxygen itself is generally colorless as a gas. However, the answer is more nuanced than a simple "colorless."
The Color of Oxygen in Different States
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Gaseous Oxygen: In its most common state – as a gas – oxygen is colorless, odorless, and tasteless. This is the form we breathe and interact with most frequently.
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Liquid and Solid Oxygen: Interestingly, liquid and solid oxygen exhibit a pale blue color. This is due to light absorption in the electronic transitions of the oxygen molecule. The reference material explicitly states this: "The liquid and solid forms are a pale blue color and are strongly paramagnetic." This pale blue color is subtle but noticeable.
Why the Color Difference?
The difference in color between gaseous and condensed (liquid and solid) oxygen comes down to the intermolecular distances and interactions. In the gaseous phase, the molecules are far apart, and light absorption is minimal. As oxygen transitions to the liquid or solid phase, the molecules are much closer together, leading to increased interaction and absorption of light, resulting in the pale blue hue.
