Because of the extremely high temperatures normally required to cause ionization, we do not often encounter plasmas in our everyday lives, although they are present, and strongly influence the interactions that take place in our Sun and other stars. The physical behavior of gases is described by the kinetic theory of gases , in which individual atoms or molecules are considered to travel at high speeds in all directions within the gas sample. The kinetic theory accounts for many physical properties of a gas, such as pressure, temperature, volume, diffusion rate, and so on. Pressure results from the frequent collisions of large numbers of gas molecules with the walls of the gas's container. The volume of a gas sample is variable because the spacing between the gas molecules can change to allow the gas to fill any size container in which it is placed.
Temperature is related to the average kinetic energy of the moving molecules of gas.
The higher the temperature, the faster the molecules must be moving. In any equation dealing with the kinetic energy of gases, absolute temperature must be used as expressed using the Kelvin scale. The pressure of the gas is also important in such calculations. Other quantities that may be specified when describing a gas sample are the number of gas molecules present, the volume, and the average speed of the individual molecules. The relationships among pressure, temperature, volume, and number of gas molecules in a sample can be expressed in two ways, using the gas law equations as given in this section of your textbook.