What does it look, feel, taste, or smell like?
Oxygen comes in two common forms, diatomic oxygen (O2) and ozone (O3), although larger molecules such as O4 and O8 can occur under very rare conditions. O2 is a colorless gas in most conditions, but when it is liquid it is blue. O3 is a blue, toxic gas with a sharp odor.
How was it discovered?
In 1772, Carl Wilhelm Scheele discovered that heating mineral oxides generated a gas that supported combustion (burning) better than air. He was able to collect a pure form of this gas for his experiments, but waited several years to publish his results.
In the meantime, in 1774, Joseph Priestley independently performed similar experiments to generate and collect oxygen gas. Like Scheele, he demonstrated oxygen's ability to support combustion better than air. In addition, he showed oxygen was able to support life in mice up to four times as long as in air. He promptly published his results and is generally credited as the discoverer of oxygen.
Where did its name come from?
Oxygen comes from Greek and means "acid forming." When oxygen was named, scientists thought all acids contain oxygen, but now we know that's not true, although most of them do (acetic acid, CH3COOH, which is the acid in vinegar, and sulfuric acid, H2SO4 or oil of vitriol, do contain oxygen, but hydrochloric acid, HCl, does not.)
Where is it found?
Oxygen is found all over the Earth, in the crust and in the atmosphere. Oxygen is also found in the water that covers most of the earth's surface.
What are its uses?
Without oxygen, life as we know it would not exist. There are only a few microbes known that do not require oxygen to live. Every other life form including tiny bacteria, fungus, trees, animals and people needs oxygen. Oxygen is crucial to our bodies — we need oxygen from the air to survive.
Ozone in the atmosphere helps shield the Earth from harmful UV rays given off by the sun.
Oxygen is used along with another gas called acetylene to power metal welding or cutting torches. Rockets carry oxygen in tanks to mix with fuel. The combustion of the fuel and the oxygen is what propels the rocket.
Scuba divers carry tanks of oxygen mixed with nitrogen that allow them to breathe under water.
It is also used in respirators in hospitals for people who can't get enough oxygen themselves (for instance, if they can't breathe on their own.)
Is it dangerous?
O2 can actually be toxic, even though we rely on it to survive. This is only true at very high concentrations of oxygen.
Ozone can be toxic because it oxidizes things even more easily than O2.
Oxygen can also help the spread of fire, so it can be dangerous for that reason.
Oxygen is very reactive, and, as a result, can combine with other elements very easily. Some of those compounds are more dangerous than the element. Oxides of nitrogen and sulfur are very dangerous.
Liquid oxygen is dangerous because of its cold temperature alone. Paradoxically it is even more of a fire hazard than oxygen at room temperature because it is more dense than not-so-cold oxygen. Objects that can burn that are placed in liquid oxygen absorb oxygen and become even more violently flammable.