Gas is ane o the fower fundamental states o matter (the ithers bein solit, liquid, an plasma). A pure gas mey be made up o individual atoms (e.g. a noble gas or atomic gas lik neon), elemental molecules made frae ane teep o atom (e.g. oxygen), or compoond molecules made frae a variety o atoms (e.g. carbon dioxide). A gas mixtur wad contain a variety o pure gases much lik the air. What distinguishes a gas frae liquids an solits is the vast separation o the individual gas pairticles. This separation uisually maks a colourless gas invisible tae the human observer. The interaction o gas pairticles in the presence o electric an gravitational fields are considered negligible as indicatit bi the constant velocity vectors in the image.

Gas phase pairticles (atoms, molecules, or ions) muive aroond freely in the absence o an applee'd electric field.