Liquid oxygen

The air we breathe consists of 78% nitrogen (N₂) and 21% oxygen (O₂). At normal temperatures, nitrogen and oxygen are gases. But if you cool nitrogen very much, lower than -196 degrees Celsius, it becomes a liquid. If you heat that liquid again, it turns back into a gas. Oxygen changes from liquid to gas at -183 degrees Celsius, which is slightly warmer than nitrogen. This allows you to separate them by using temperature.

When you pour liquid nitrogen into a can, the can becomes very cold. So cold that the oxygen gas from the air around the can turns into liquid. The liquid oxygen forms droplets on the outside of the can, just like you see water droplets on a cold mirror after a hot bath. These oxygen droplets are collected in a container kept cold by liquid nitrogen, so they do not turn back into gas. Sometimes some liquid nitrogen is included, but it evaporates first because nitrogen has to be colder to remain liquid. In the end, all you are left with is liquid oxygen.

Oxygen is part of the fire triangle: fuel, oxygen, and heat. If you add oxygen, you can make a fire bigger. For example, if you put burning matches in liquid oxygen, you get a much bigger flame because there is a lot of oxygen. We call this an oxidation reaction.

In factories, liquid oxygen is made by distillation. First, all other things, such as CO₂ and dust, are removed from the air. Then the air is made very cold until it liquefies. The mixture is then slowly heated so that the nitrogen evaporates first and the liquid oxygen remains.