Ideal Gas Law

Abstract The kinetic theory of gases established that invisible gas molecules are ceaselessly and rapidly moving. Gas pressure is a momentum flux density measured by interaction with surfaces. Motion does not cease at absolute zero because at sufficiently low temperatures all real gases liquefy. By definition the internal energy of an ideal gas is independent of density. Air is mostly empty space. Intermolecular collisions ensure local thermodynamic equilibrium and determine mean free paths of at least thirty molecular diameters. Molecular speeds are distributed about a mean, a decrease of about 7% corresponding to a temperature drop from 20∘C to −20∘C. Relatively fewer molecules drive chemical reactions. Even fewer molecules have sufficient energy to escape Earth’s gravity. Pressure and density decrease exponentially with height in the troposphere with a scale height of around 8 km because of gravity. Earth’s atmosphere is well-mixed up to about 100 km.