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Why do gases fill the entire container that holds them?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

Gases fill the entire volume of any closed container because they consist of a vast number of molecules that move chaotically in all directions at high speeds. Unlike solids, which have a definite size and shape, or liquids, which maintain a specific volume, gas molecules are separated by distances large relative to their size and do not remain in a fixed position.213

According to the kinetic theory of gases, these molecules travel in arbitrary, random trajectories, colliding with one another and with the walls of the container. Because there are no strong attractive forces—such as those found in condensed states—to hold the molecules together in a specific volume, they distribute themselves randomly throughout the available space. The pressure exerted by a gas is simply the result of these innumerable, constant molecular impacts against the container walls.2134