Diffusion is a fundamental physical process observed in liquids and gases. It is defined as the net movement of particles from a region of their higher concentration to a region of their lower concentration, as a result of their random movement. This process is passive, requiring no external energy input, and is a direct consequence of the intrinsic kinetic energy of particles.

According to the kinetic particle theory, particles in a fluid (a liquid or a gas) are in a state of continuous, random motion.

1. Random Motion: Particles move in straight lines until they collide with other particles or the walls of their container. These collisions cause them to change direction randomly.
2. Probability and Net Movement: In an area of high concentration, there are many particles. In an area of low concentration, there are few. Statistically, there is a higher probability that a particle will move from the crowded region to the less crowded region than vice versa.
3. Equilibrium: This "net movement" continues until the particles are evenly distributed throughout the available space. At this point, the concentration is uniform, and while individual particles continue to move randomly, there is no further net change in concentration.

The velocity at which diffusion occurs is influenced by the physical state of the environment and the nature of the particles themselves.

1. Temperature

Temperature is a macroscopic measure of the average kinetic energy of a population of particles. As the temperature of a fluid increases, its particles move faster and collide more frequently. This increased activity leads to a more rapid dispersal, thereby increasing the rate of diffusion.

2. Physical State

Diffusion occurs most rapidly in gases because gas particles have higher kinetic energy and are much further apart than liquid particles, meaning there is less resistance to their movement. Diffusion is slower in liquids because the particles are closer together and experience significant attractive forces. Diffusion does not occur in solids because the particles are locked in fixed positions.

3. Molecular Mass

At a given temperature, all particles have the same average kinetic energy. According to the formula for kinetic energy ($E_k = \frac{1}{2}mv^2$), lighter particles ($m$) must have a higher velocity ($v$) to have the same energy as heavier particles. Consequently, molecules with a lower relative molecular mass diffuse faster than heavier molecules.

Diffusion is the primary mechanism by which substances move and mix in the natural world. By explaining this phenomenon through the lens of kinetic particle theory, we can understand why smells travel through a room, why tea disperses in hot water, and how essential gases move across biological membranes.