Why Does Diffusion occur?
Diffusion is a common term applied to liquids as well as gases. This is a phenomenon where there is a movement of molecules passively from the region of their high concentration to a region of low concentration. Osmosis is a phenomenon in which the solute molecules in a solution move from its low concentration to its high concentration across a semi permeable membrane. When the biological cell has to create balance between the inside of the cell and the extra cellular space, then the passive movement of the particles occurs across the partial or semi permeable membrane. The semi permeable membrane allows only some particles to move across it and not all.
When the solute concentration is more inside the cell and is lesser outside, water or solvent moves from the extra cellular space to the inside of the cell. This type of maintaining balance is done by the water molecules. If there is any movement of molecules randomly from the low solute concentration to the higher concentration, then it is called diffusion. This can also be written as the movement of solvent molecules from their higher concentration to a lower one to attain equal concentration on both sides. This movement is spontaneous and random. The difference in solvent concentration is called as concentration gradient. Diffusion is always a movement down the concentration gradient. It happens very spontaneously across higher concentration gradient. This phenomenon occurs due to the movement of molecules as per the kinetic theory of gases. In diffusion, the solute molecules tend to get distributed in equal concentration all over. It also tends to take place at high temperatures. Diffusion is not similar to the movement of molecules in the flowing waters. In the flowing water, the movement of molecules will not be random but the molecules all move at a time in a single direction.
So, the particles move randomly with a change in temperature by getting distributed from the center of particle concentration. This process is what occurs in diffusion and is the reason behind the particle movement.