Yes, air is a mixture. It contains several different gases that are not chemically bonded to each other, each retaining its own properties. The two main components are nitrogen at 78% and oxygen at 21% by volume, with the remaining 1% made up of argon, carbon dioxide, and small amounts of other gases.
What Air Is Made Of
Dry air has a remarkably consistent recipe. Nitrogen makes up 78.084% of the atmosphere by volume, oxygen accounts for 20.946%, and argon fills most of the remaining space at 0.934%. Carbon dioxide, despite its outsized role in climate, represents just 0.042% of dry air. As of late 2024, global carbon dioxide levels measured about 427 parts per million, a number that has been climbing steadily for decades.
Beyond these major players, air contains trace amounts of neon, helium, krypton, and xenon. Water vapor is the wild card: it ranges from nearly 0% in arid deserts to about 4% by volume in hot, humid tropical regions. Because water vapor fluctuates so much, scientists often describe air’s composition in “dry” terms to keep the baseline consistent.
Air also carries tiny solid particles and liquid droplets, collectively called particulate matter. Dust, pollen, soot, smoke, and volcanic ash all float in the atmosphere. These aren’t gases, but they’re part of the air you actually breathe.
Why Air Is a Mixture, Not a Compound
In chemistry, a compound is a substance where atoms of different elements are locked together by chemical bonds in a fixed ratio. Water is always two hydrogen atoms bonded to one oxygen atom, no matter where you find it. A mixture, by contrast, is a collection of different substances that sit together without bonding. Each component keeps its own chemical identity.
Air fits the definition of a mixture for three clear reasons:
- No fixed ratio. The proportions of gases in air can vary. Water vapor swings between 0% and 4%, and carbon dioxide levels differ between a busy city intersection and a remote mountaintop. A compound always has the same ratio of elements in every sample.
- Components keep their properties. Oxygen in the air still behaves like oxygen. It supports combustion, reacts with metals, and can be breathed by your lungs. Nitrogen still behaves like nitrogen. Neither has been transformed into something new the way hydrogen and oxygen are transformed when they form water.
- Physical separation works. You can pull air apart into its individual gases without any chemical reaction. That would be impossible with a compound, where you’d need to break chemical bonds.
Homogeneous, Not Heterogeneous
Mixtures come in two varieties. A heterogeneous mixture has visibly different regions, like a bowl of trail mix where you can pick out the individual nuts and raisins. A homogeneous mixture looks uniform throughout because its components are evenly distributed at the molecular level.
Clean air is a homogeneous mixture. If you took a sample from one corner of a room and another from the opposite corner, both would contain the same proportions of nitrogen, oxygen, and argon. You can’t see or feel boundaries between the gases. That said, air becomes somewhat heterogeneous when it’s loaded with visible particulate matter like smoke or dust clouds, since those particles aren’t evenly distributed.
How Air Can Be Separated
The fact that air can be separated using physical methods is one of the strongest pieces of evidence that it’s a mixture. The industrial process is called fractional distillation. Air is first cooled below negative 200°C, which turns it into a liquid. Then the liquid air is slowly warmed. Because each gas has a different boiling point, they evaporate one at a time: nitrogen boils off at negative 196°C, and oxygen at negative 183°C. Each gas is collected separately as it evaporates.
This process produces the pure oxygen used in hospitals, the nitrogen used in food packaging, and the argon used in welding. No chemical reaction is needed, just temperature changes. If air were a compound, you’d need to break molecular bonds to get those individual gases out.
Does Air’s Composition Change With Altitude?
Air gets thinner as you go higher, which is why it’s harder to breathe on a mountaintop. But the relative proportions of the major gases stay essentially the same throughout the lower 80 kilometers of the atmosphere. A breath of air at sea level and a breath at 10,000 meters contain the same percentage of oxygen and nitrogen. There’s just less total air in each breath at higher altitudes.
The one notable exception is the ozone layer, roughly 20 to 40 kilometers above the surface, where ozone (a molecule made of three oxygen atoms) is found in higher concentrations than elsewhere. This doesn’t change the overall nitrogen-to-oxygen ratio in any meaningful way, but it does create a localized pocket where the composition isn’t perfectly uniform.