Diffusion is the movement of particles from an area of high concentration to an area of low concentration. This occurs because particles are in constant motion and will spread out over time if there is nothing to stop them.

In a gas, diffusion occurs because the particles are not held together by any strong forces. This means that they can move around freely and will spread out until they reach an equilibrium, where the concentration of particles is the same throughout the gas.

Diffusion is an important process in gases because it allows for the mixing of different gases. This is essential for many processes, such as combustion and respiration.