Gases are the most diffuse form of matter. In a gas the molecules (the smallest particles of which the gas is made--for some gases these equate to atoms) are highly energetic and they can be widely separated from each other. The behavior of gases and their properties derive from these facts.

Sometimes the term vapor is used to describe a gas. Strictly speaking a gas is a substance at a temperature above its boiling point. A vapor is the gaseous phase of a substance that, under ordinary conditions, exists as a liquid or solid.

One of the more obvious characteristics of gases are their ability to expand and fill any volume they are placed in. This contrasts with the behavior exhibited by solids (fixed shape and volume) and liquids (fixed volume but indeterminate shape). Gases have an indeterminate volume and...