Gases

The gaseous state is a unique state of matter. In this state, the individual molecules that make up the gas are much further apart than in either the solid or liquid states. Thus, gases are the least dense form of matter. In practical terms, this means that if a fixed quantity of gas is cooled to the liquid state, the volume will decrease dramatically.

This can be demonstrated by filling an aluminum can with steam, then inverting this in a bucket of cold water. Inside the can, the steam cools and turns into liquid water. Since the volume of material in the can has decreased, the pressure drops. Atmospheric pressure crushes the can.

Following are several examples of "trapping" of gases.

Helium balloons (which obviously contain a gas different than "air")
Rising of dough (bread, cakes, etc.)
Foams, such as styrofoam cups

Our atmosphere is composed of a mixture of several different gases. The following table is taken from "Chemistry in Context", published in 2000 by McGraw-Hill for the American Chemical Society. Since air contains varying quantities of water vapor (depending on the relative humidity), the values listed are for "dry air".

Nitrogen (N₂)	78%
Oxygen (O₂)	21%
Argon	0.9%
Carbon Dioxide (CO₂)	0.04%

Nitrogen, argon, and carbon dioxide are all unreactive gases. However, oxygen is an important starting material for the following reactions.

Combustion - burning of most fuels requires oxygen and produces carbon dioxide.
Rust - rusting is a slow reaction of metal with oxygen.
Metabolism - the body requires oxygen to produce energy. This process also produces carbon dioxide.
Ozone - the primary method for production of ozone in the upper atmosphere starts with oxygen.