Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

A closed bottle of perfume contains a high concentration of gaseous aromatic molecules that are constantly moving and randomly colliding. Meanwhile, the air outside of the bottle contains essentially none of these molecules.

On opening the bottle, a concentration gradient is established between these high- and low-concentration regions. The molecules continue moving randomly, with overall movement from the high-concentration region to the low-concentration region.

The spontaneous mixing and spreading of liquids or gases in response to a concentration gradient is called molecular diffusion.

Diffusion is a slow process. Even though gas particles travel at high speeds, the numerous collisions cause frequent changes in speed and direction.

The average distance a particle travels between collisions is known as its mean free path. For a gas particle, its mean free path is influenced by the particle density, which also affects the pressure.

As the particle density increases, so does the collision frequency. Thus, their mean free path is shorter. Likewise, as the particle density decreases, so does the collision frequency, leading to a longer mean free path.

Different gases diffuse at diff