Diffusion is caused when heat in a gas or water environment creates energy that is absorbed by particles, making them move. This is referred to as thermal motion. Each particle moves in a different direction, but the direction changes if the particle hits a solid object or another particle. This causes each particle to move away from an area that is highly concentrated.
As particles move from concentrated areas, they diffuse throughout the liquid or gas to create an equilibrium, or even spacing. The rate of diffusion is dependent on three aspects: the temperature, the mass of the particles and the thickness of the liquid or gas. An increase in temperature creates more energy, so the particles move faster. Heavier particles require more energy to move; the higher the mass, the slower the movement. Particles move the fastest through gas and the slowest through a solid. There may be highly limited movement through a solid approaching cold enough temperatures. Movement through liquid is faster than movement through a solid, but not as fast as through gas. This is due to the molecules in the particle maintaining a weak bond with the molecules in the liquid. Until another particle or solid object is hit, a particle will move in a straight line.