Why does diffusion occur

Diffusion is a term that we all come across during our school science classes. In very simple terms, it is the process due to which a solution tries to attain concentration equilibrium. This means that the solution is naturally inclined towards a composition where the solute molecules are evenly spread on the solvent, which is water in most cases. The concentration of the solute molecules is not uniform in a solution. There is an area with higher solute concentration than the surroundings. For instance, if in a cell, the internal space is solute rich whereas the extra cellular membrane does not have an equivalent amount of solute molecule concentration, then there would be a spontaneous flow of solvent from the external area to the insides of the cell, through the semi permeable or partial membrane separating the two spaces. This process is called diffusion. The phenomenon of diffusion occurs in liquids and gases.

Diffusion is actually the means through which a cell maintains the balance of concentration between the insides of the cell and the extra-cellular area. Water molecules in the external part of the cell diffuse through the semi permeable cell membrane and result in the increase of the solute concentration of that part. Also, the solute molecules inside the cell membrane diffuse towards the external area. These cumulative effects comprise the overall diffusion mechanism.

The basic premise behind the tendency of solutions to attain concentration equilibrium by overcoming the concentration gradient (the discrepancy in the concentration at different areas)is molecular motion. The random motion of the molecules is attributed to their inherent kinetic energy. There is a correlation between kinetic energy and temperature of the solution. Higher temperatures fuel greater molecular motion and lead to greater probability of diffusion. Hence, the diffusion phenomenon is favoured by high temperature.

The high degree of randomness in the motion of the water molecules (or solvent molecules) differentiates it from the molecular motion of a flowing fluid. In the latter case, there is at least some sense of predictability about the motion patterns, which does not apply to a solution undergoing the diffusion mechanism.