continuum gas

Continuum gas refers to a gas where molecular collisions with other gas molecules are more significant than collisions with surfaces. This is in contrast to a free-molecule gas, where molecules primarily interact with surfaces. In continuum gases, the mean free path—the average distance a molecule travels between collisions—is small compared to the physical dimensions of the container or system.1

Gases, in general, lack a definite size or shape, filling any container they occupy. Their properties are dependent on the container's volume, not its shape. There is no fundamental distinction between a gas and a liquid; they represent different states of a fluid, with the critical temperature defining the point above which no phase change occurs. The behavior of gases at low pressures or in small spaces can transition towards free-molecule effects.123