The word gas, which was first used in the middle of the 17th century by the Belgian physician Johann van Helmont, was derived rom the Greek word chaos. Apparently he chose chaos because so many of the vessels he used exploded when he tried to propare gases by chemical reactions. The study of gases in the latter part of the 18 cenrury and the carly par of the 19 century resulted in significant developments in chemistry, Although gases differ as widely in their chemical behavior as do licuids and solids, their physical behavior is remarkably uniform Gases fill whatever space is available, expand and contract with changes in temperature, exert greater pressure with increasing tempemture, flow rapidly, are compressible and generally have low density. This similarity in physical behaviar suggests compressible and generally have low that these properties are not independent of one another and can be related by a theorotical model. Gases consist of particles which are in constant random motion and which collide with one another and with the walls of the vessel Since gas particles move at high velocity and are sepatuted by large distances, the forces acting between them(intermolecular forces tend to be negligible. It is for this reason that gases are free to expand to fill the volume of any container