Oxygen (O), from the greek roots, oxy- for acid, and gen- for generator (oxygen was erroneously thought to be responsible for acidity, as opposed to hydrogen) is a quintessential element that makes up a significant percentage of the matter in the universe. Together with hydrogen it creates water, which makes up over 70% of our body mass. It exists stably as a diatomic gas, O₂. Our atmosphere contains approximately 21% oxygen, with the remainder being almost exclusively nitrogen.

Oxygen can be isolated from water if done with a proper gas capture electrolysis setup.

A 12 volt car battery works well for this application. Use distilled water with a bit of table salt added to make it conductive. The vials should be full of water to begin to ensure no atmosphere is present. Oxygen will be removed from the hydrogen tube automatically, but nitrogen, which composes 78% of our atmosphere, will remain.

Another method is to decompose hydrogen peroxide. Fill a small container completely with it. Extract some maganese dioxide electrolyte paste from a heavy duty battery (see carbon) and add a small amount of this to the peroxide. It will begin to bubble oxygen gas rapidly. Place a (ideally clear) balloon over the jar to capture the gas (hence needing a small container). You can also capture the gas with an upside-down vial that is filled with peroxide or water and allowed to be displaced by the oxygen bubbles.

The peroxide will generate 10x its volume in oxygen gas. Wear gloves when handling the electrolyte, as it is caustic, and will stain everything black.

Pure oxygen will cause an ember (from a burnt out matchstick for example) to burst into flame.