What is effusion in the context of gas behavior?

Effusion refers specifically to the process where gas particles escape from a container into a vacuum or another space through a small opening or orifice, typically much smaller than the dimensions of the gas particles themselves. This movement occurs from an area of higher pressure to an area of lower pressure, which aligns perfectly with the definition provided in the context of the correct answer.

In effusion, the speed and mean free path of gas molecules play a crucial role, as they determine how frequently gas particles can collide with and pass through the tiny opening. This process is different from diffusion, which involves the mixing of gases due to their random motion and is influenced by factors such as concentration gradients and temperature.

Understanding this distinction clarifies why the movement of gas through a pinhole from high to low pressure is accurately described as effusion, as it captures the core mechanism of this behavior. The other options describe different phenomena related to gas behavior, but they do not align with the specific definition of effusion. For example, spreading of one gas into another typically pertains to diffusion, while absorption involves gas dissolving into another medium rather than passing through an opening.