Diffusion is the movement of molecules which spread out and occupy any available space. It can also be described as the passage of fluid from a region of high molecular concentration to one of lower molecular concentration. It is a consequence of kinetic properties of elements of matter.
It occurs in fluids - liquids and gases- whereby molecules move freely and become evenly distributed by moving through available space. It doesn’t happen in solids due to their fixed nature. Solids are held together by the force of attraction which prevents them from moving freely.
After a substance has diffused completely into the available space, the molecules will continue moving around however without net movement. This is what is called dynamic equilibrium where there is the lack of a concentration gradient.
The rate of diffusion depends on different factors.
Molecules always move randomly depending on the environment, their mass, the amount of thermal energy which a function of temperature. With an increase in temperature, the kinetic energy increases. Which increases velocity. Therefore, the higher the temperature, the higher the rate of diffusion. Similarly, the lower the temperature, the rate of diffusion will also be low.
Distance also has a significant impact on diffusion. In that case, the greater the distance a substance has to move, the slower the rate of diffusion.
Other factors also have a significant impact on the process of diffusion in living systems.
Molecular weight affects the rate of diffusion because of the bigger the size of the molecule the longer it takes to diffuse. Membrane size is another variable that affects the rate of the rate of diffusion. If the membrane’s pores are small, it will take molecules longer to diffuse through it, than if the pores were larger. Also, the layer thickness is a variable because of the thinner the membrane, the quicker the diffusion.