Air, Oxygen, and Ozone
Air is a mixture, oxygen is one of its important components, and ozone is a different molecular form of the same element.
1. Air is a mixture of gases
Clean dry air near the surface contains approximately 78% nitrogen, 21% oxygen, 0.9% argon, and about 0.04% carbon dioxide by volume, plus very small amounts of other gases. Water vapor is not included in the dry-air values because its amount changes strongly with conditions.
| Component | Formula | Approximate volume fraction in dry air | Role or property |
|---|---|---|---|
| Nitrogen | \(N_2\) | 78% | Relatively unreactive under ordinary conditions; dilutes oxygen |
| Oxygen | \(O_2\) | 21% | Supports respiration and many oxidation reactions |
| Argon | \(Ar\) | 0.9% | Very unreactive noble gas |
| Carbon dioxide | \(CO_2\) | about 0.04% | Used in photosynthesis and involved in heat balance |
Because the particles are not chemically bonded into one substance, air keeps the characteristic properties of its components and can be separated by physical methods. Its composition also varies: humid air contains water vapor, and local dust or pollutants are not permanent components of clean dry air.
2. Oxygen, \(O_2\)
At ordinary conditions oxygen is a colorless, odorless gas. Each molecule contains two oxygen atoms. Oxygen is not itself a fuel, but it is an oxidizing agent: it accepts electrons in many reactions and allows many fuels to react rapidly. A glowing wooden splint relights in oxygen, which is a common laboratory test. The test must be performed on a collected sample with proper laboratory supervision; oxygen must never be identified by smelling it.