The state of a thermodynamic system, defined in terms of the pressure, volume, and temperature, depends on the statistical behavior of a large number of particles. The statistical behavior of thermodynamic systems is the domain of kinetic theory.

Pressure, for example, measured as the force per unit area, is exerted on a surface as the result of an enormous number of collisions. Likewise, the temperature of an ideal gas increases with the average kinetic energy of an enormous number of particles.

To understand pressure and temperature, keep a clear sense both of the statistical nature of gaseous systems and a sense of the basic mechanics at play at the level of the particles. Concepts from momentum & impulse and work & energy are crucial not only to understanding the behavior of gases at the particle level but how that behavior determines the thermodynamic macrostate.