The Gas Laws told you how a gas behaves. This page is about why it behaves that way, and then about the particular mixture of gases you have been breathing throughout — which is the largest chemical system any of us will ever be part of, and one that a handful of trace components run.
States of matter are a contest between two things
Every substance can be a solid, a liquid, or a gas, and which one it is at a given moment is decided by a competition:
- the kinetic energy of the particles, which increases with temperature and pushes them apart;
- the forces of attraction between the particles, which pull them together and do not depend on temperature at all.
In a solid, attraction wins outright. Particles are held in fixed positions and can only vibrate about them. In a liquid, kinetic energy is enough to let particles slide past each other but not enough to let them escape, so a liquid keeps its volume but not its shape. In a gas, kinetic energy wins completely: particles have escaped one another entirely, move independently, and fill whatever they are put in.
The strength of those attractions differs enormously between kinds of substance, and melting and boiling points are the direct evidence. In an ionic solid the attractions run between full charges throughout a lattice, which is why sodium chloride melts at 801 °C. In a molecular substance the attractions are between whole molecules and are far weaker, which is why methane boils at −161 °C — and why water, whose molecules hold each other with hydrogen bonds as Water and Solutions describes, boils some 260 degrees higher than methane despite being the smaller molecule.
This is also why gases are compressible and liquids and solids are essentially not. A gas is mostly empty space, so there is somewhere for the particles to go. In a liquid they are already touching.
The kinetic molecular theory
The model behind all of it makes five assumptions:
- A gas consists of particles in constant, rapid, random motion, travelling in straight lines until they collide.
- The particles are extremely small compared with the distances between them — a gas is mostly empty space, and the particles’ own volume is negligible.
- Collisions between particles, and with the walls, are perfectly elastic: no kinetic energy is lost.
- There are no forces of attraction or repulsion between the particles.
- The average kinetic energy of the particles is directly proportional to the absolute temperature.
Everything measurable about a gas falls out of those five statements.
Pressure is the total force of countless particle collisions with the container walls, divided by the area. Nothing else. Temperature is the average kinetic energy of the particles — “average” being important, since at any instant some particles are moving far faster than others.
The gas laws follow directly. Halve the volume and each particle reaches a wall in half the distance, so collisions are twice as frequent and the pressure doubles — that is Boyle’s law, derived rather than measured. Raise the temperature at constant pressure and the particles hit harder, so the container must expand until the collision rate falls back — that is Charles’s law. Diffusion, the spread of a smell across a room, is assumption 1 in action, and the fact that lighter particles diffuse faster follows from assumption 5: at the same temperature the average kinetic energies are equal, so the lighter particles must be moving faster.
Assumptions 2 and 4 are the two that are false, and they are precisely the two that fail at high pressure and low temperature — which is exactly where The Gas Laws said the ideal gas law breaks down. The model tells you where the model stops.
What the air is made of
Dry air, by volume, near the surface:
| Gas | Formula | Percentage by volume |
|---|---|---|
| Nitrogen | 78.08 | |
| Oxygen | 20.95 | |
| Argon | 0.93 | |
| Carbon dioxide | about 0.04, and rising | |
| Neon | 0.0018 | |
| Helium | 0.00052 | |
| Methane | about 0.00019 | |
| Krypton | 0.00011 |
Three things in that table are worth more than the numbers.
“Dry” is a real qualification. Water vapour is left out because it is not constant — it varies from near zero over a cold desert to around 4% in humid tropical air, and it changes hour by hour where you live. Everything else in the table is remarkably uniform worldwide, which is itself a finding: the atmosphere is well mixed.
Argon at 0.93% is the third most abundant gas, ahead of carbon dioxide by more than twenty times, and almost nobody can name it. It is there because it is inert — it takes part in nothing, so nothing removes it.
The trace gases do the interesting work. Carbon dioxide is about four parts in ten thousand and is the reason the planet’s surface is habitable rather than frozen; a small proportional change in it is a large change in the energy balance. Ozone, which is a vanishing fraction of the whole, absorbs the ultraviolet radiation that would otherwise reach the surface. Being present in small amounts and being unimportant are completely different things, and the atmosphere is the clearest example of it.
By Avogadro’s hypothesis these percentages by volume are also percentages by number of molecules — which is why an atmospheric chemist can move between “0.04% by volume” and “about 420 parts per million” without doing any conversion beyond shifting a decimal point.
Air quality, and why small numbers matter
The pollutants that air quality reporting tracks are present at parts per million or parts per billion, and they matter at those levels.
- Ground-level ozone is not the same ozone as the protective layer in the stratosphere. It is the same molecule doing harm in the wrong place, formed near the ground when nitrogen oxides and other pollutants react in sunlight. It irritates airways and is worst on hot, still, sunny days.
- Fine particulate matter is solid and liquid particles small enough to be drawn deep into the lungs. Incomplete combustion is a major source, as Combustion set out, along with wildfire smoke, which travels much further than most people expect.
- Nitrogen and sulfur oxides come from burning fuels, dissolve in atmospheric moisture, and fall as acid precipitation. That is the non-metal-oxide chemistry from Acids and Bases happening on a continental scale, and it was severe enough in the Great Lakes region that reducing sulfur emissions became the subject of agreements between Canada and the United States.
- Carbon monoxide is the incomplete-combustion product that is hazardous indoors at levels far below anything visible.
Canada reports these together as the Air Quality Health Index, a scale that combines ozone, fine particulate matter, and nitrogen dioxide into a single number with health advice attached. It is worth looking up your own region’s current value, because it makes the abstraction concrete — and because the number changes with the weather in ways this unit lets you explain.
The stratospheric ozone story is the useful counterweight to all of this. Chlorofluorocarbons used as refrigerants were found to destroy stratospheric ozone; the Montreal Protocol phased them out internationally; the ozone layer has since begun to recover. It is a real case of chemistry identifying a problem, quantifying it, and international agreement fixing it — which is exactly the kind of question The Air We Share asks you to argue about.
Once you can put a number on the gas as well as name it — how many moles of carbon dioxide a given volume contains, how much a reaction will produce — you are doing the same Stoichiometry as before with one more door onto the road. That is the last thing this course asks of you before The Chemistry Showcase.
Curriculum connection
F3.1
identify the major and minor chemical components of Earth’s atmosphere
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F3.2
describe the different states of matter, and explain their differences in terms of the forces between atoms, molecules, and ions
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F3.3
use the kinetic molecular theory to explain the properties and behaviour of gases in terms of types and degrees of molecular motion
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