n.
One of the three fundamental states of matter , in which matter has no definite shape, is very fluid, and has a density about 0.
1% that of liquid s. Gas is very compressible but tends to expand indefinitely, and it fills any container. A small change in temperature or pressure produces a substantial change in its volume; these relationships are expressed as equations in the gas laws . The kinetic theory of gases , developed in the 19th century, describes gases as assemblages of tiny particles ( atom s or molecule s) in constant motion and contributed much to an understanding of their behaviour. The term gas can also mean gasoline , natural gas , or the anesthetic nitrous oxide . See also solid .