What is a Gas?
AIR. Gases show little or no cohesive force and may be thus differentiated from liquids and solids. Heating any substance reduces the cohesive force, and theoretically, it is possible to change any substance to a gas by bringing it to a sufficiently high temperature. In general usage the term, gas, is limited to those substances which have the gaseous form under usual conditions of temperature and pressure. The prime physical characteristic of a gas is that it uniformly fills any vessel in which it is placed regardless of the vessel's shape or size. Liquids conform to the shape of the vessel only so far as they can fill it, and are bounded at the top by a liquid surface. The shape of solids is independent of that of the container.