But some gases have a color, or an odor, or both. For example, nitrogen dioxide is brown. Hydrogen sulfide smells like rotten eggs. Under special conditions, gases change into a fourth state of matter called a plasma. Plasmas are formed by heating a gas to an extremely high temperature or by passing an electric current through it.
Matter exists in a plasma state in stars and the regions between stars.
How gases behave. The behavior of gases is explained by what scientists call the kinetic theory. An atom is one of the basic units of matter, and a molecule is a combination of atoms. The number of atoms or molecules of gas in a container the size of a pin-head is many millions of times as large as the number of people on the earth. But these gas particles are so small that they occupy only about one-thousandth of the space inside the container.