Oxygen

Today we know that oxygen makes up a fifth of the air and is the critical element that lets us breathe. But how did we figure that out?

In 1668, English chemist John Mayow published results of the experiments he conducted on the study of air. He made some extraordinary insights. In one experiment, he placed either a burning candle or a live mouse in a glass, which he set upside-down in a wider vessel containing water, thus trapping the air. The candle would go out and the mouse would die. In each case, the water level rose within the inverted glass, taking up the space of something that had been consumed...

Some of this had been noted before by the likes of Leonardo da Vinci and even the ancient Greeks, but Mayow observed that, each time, the rising water displaced the same exact volume of air. That was a big clue. He figured out that one part of whatever made up the air was being consumed by the flame and the breathing mouse. He correctly calculated that the component in question amounted to one fifth of the air!

He also noted that when heated, the ore antimony would get heavier. He figured that it was bonding with that mysterious component of air. He even figured out that the purpose of the lungs is to extract this specific air component, reasoning that blood would then shuttle the component around the body to power our muscles and generate body heat. He was absolutely right! But it would be another century before scientists actually identified oxygen.

About a hundred years later in the 1770s, a trio of European scientists made the leap to identify oxygen as an element. In Sweden, Carl Wilhelm Scheele obtained oxygen by heating mercuric oxide. He determined that the emerging gas was the same one that was required for combustion. In England, Joseph Priestly also heated mercuric oxide, by focusing sunlight on it. He found that the emerging gas made candles burn brighter and made breathing easier. Mice breathing the gas became more active, and he personally felt “light and easy” for a while after breathing it.

Both Scheele and Priestly wrote to share their work with Antoine Lavoisier, in France. Lavoisier repeated several of Priestly’s experiments. He also noted that oxygen formed acids when combined with nonmetals, such as sulfur, phosphorus, charcoal, and nitrogen. He proposed naming it oxygen, from the Greek for acid (oxy) forming (gen). Soon thereafter, he helped determine that water was not an element but rather a blend of the elements hydrogen and oxygen.

This understanding laid the foundation for modern chemistry and our knowledge of how air and chemical reactions work. Oxygen plays a crucial role in industries like medicine, metalworking, and space exploration, transforming our knowledge of biology, chemistry, and the environment.

As a gas, oxygen is colorless, odorless, and tasteless. As a liquid or solid, oxygen is a pale blue color.

As an element, oxygen is extremely reactive. It readily forms hundreds of thousands of organic and inorganic compounds and bonds directly with most other elements.

A little over half of all oxygen produced commercially is used in the manufacture of steel. Oxygen is injected into molten iron to remove unwanted sulfur and excess carbon as SO₂ and CO₂.

Another quarter of the commercial oxygen supply is for the production of chemicals used to make products such as polyester, which is then used to make plastics and fabrics.

The remaining supply is used for a variety of applications—it is needed for welding and cutting metals; as a rocket propellent; for treatment of respiratory ailments; and for life support systems in aircraft, spacecraft and space suits, submarines, and scuba diving.

In addition to gaseous oxygen as O₂, oxygen also forms another gas, ozone (O₃). Ozone is formed by an electrical discharge or ultraviolet light acting on O₂. Ozone's presence in the upper atmosphere helps prevent harmful ultraviolet rays of the sun from reaching the earth's surface. Ozone has a bluish color. Liquid ozone is bluish black, and solid ozone is violet-black. 

Oxygen is the third most abundant element in the universe. It is second most abundant on earth, just slightly behind iron, a much heavier element that makes up most of the planet’s core. Oxygen is by far the most abundant element in the earth’s crust, making up 46 percent of its mass in the form of oxides of other elements. It makes up 89 percent of the mass of the ocean and 65 percent of the human body. By volume, diatomic oxygen gas, or O₂, makes up 21 percent of our atmosphere. Oxygen is plentiful.

Fractional distillation is used to isolate oxygen for commercial and other purposes. Air is liquefied and then heated. At different temperatures, different atmospheric components will evaporate, so oxygen (and other gases) can be separated, recondensed, and collected. Another method involves flowing air over a specialized microporous material (zeolite) that absorbs nitrogen, leaving mostly oxygen.