Liquid oxygen is blue and magnetic for the same reason

Liquid oxygen is pale blue and is pulled toward magnets because each oxygen molecule carries two unpaired electrons, which make it magnetic and let it absorb red light. Oxygen gas is colourless, but below its boiling point of about −183 °C (90 K) it condenses into a liquid with a clear sky-blue tint.

The magnet trick is a classic lecture demonstration. Pour liquid nitrogen between the poles of a strong horseshoe magnet and it runs straight through; pour liquid oxygen and it collects and hangs between the poles. Oxygen is paramagnetic: its unpaired electrons act like tiny magnets that line up with the field.

The colour match with the sky, though, is a coincidence. The sky looks blue because air molecules scatter short-wavelength blue sunlight more than other colours, as NASA explains. Liquid oxygen is blue for the opposite reason: it absorbs red light at around 630 nanometres, leaving blue behind. Unusually, that absorption takes two oxygen molecules sharing one photon, which is why the colour shows up in the crowded liquid rather than in thin air.

The liquid is also a concentrated oxidiser. It expands about 860 times as it warms to room temperature, so a single millilitre holds as much oxygen as nearly 4 litres of air. That density is why it serves as a rocket propellant, and why Lawrence Berkeley National Laboratory warns that it can make ordinary combustible materials catch fire. A little over half of all commercially produced oxygen is used in steelmaking.

The first successful liquefaction of air’s components came in 1883 in Kraków, where chemist Karol Olszewski and physicist Zygmunt Wróblewski used parts of Louis Cailletet’s apparatus that Wróblewski had brought from Paris, an achievement the Jagiellonian University counts as a substantial step for cryogenics.