When compressed enough so that its molecules are as close to each other as those in a liquid, a gas is indistinguishable from a liquid. While the distinction between a liquid and its gaseous vapor is important thermodynamically, they both exhibit fluid behavior on a macroscopic scale. The dominant property of fluids is their propensity to flow spontaneously within their containers. They have to be stored in containers to prevent their moving off to other locations, in contrast to solids which need not be so constrained. The volume of a liquid is conserved when it is moved from one storage vessel to another, but a gas always expands to fill its container since its molecules are free to move about.
Fluids have no ability to hold a shape independent of their surroundings.
This property of fluids is a direct consequence of the inability of the intermolecular forces to maintain an unchanging angular orientation of the molecules with respect to each other. Fluid molecules that are close to each other at one instant can move far apart with relative ease. Many fluids are mixtures of several chemical species, such as air which is composed of nitrogen, oxygen and many trace species. Liquids can be solutions of solute species dissolved in a solvent, such as sea water. Mixtures like these still retain the characteristics of fluids, but have the added feature that the proportions of the constituents can be separately measured and may vary from point to point within the fluid.