The tendency of the molecules of a liquid solid or gas to get evenly distributed throughout the available space is called diffusion. We will study some examples.

Place a copper sulphate crystal in a beaker containing water. The crystal gradually dissolves and one can see particles of copper sulphate migrating in all directions in water.

Fix a porous partition at the base of a ‘ V’ tube. Fill one arm with water and the other with sugar solution. Slowly sugar solution migrates to the arm con­taining water. After sometime the concentration of solution in both the arms will be same.

Bum an agarbathi or open a bottle of ammonia in one comer of the room. After sometime it could be smelt even in the fartherest comer of the room. The three examples clearly illustrate the tendency of the molecules to evenly distribute themselves in the available space.


The tendency of the molecules to migrate is because of the kinetic energy present in them. When more mol­ecules are present, they collide with one another and get deflected in the direction of least resistance or concentration. After some time the diffusion rate slows down, because equilibrium has reached.

Even at equilibrium there will be some movement, but the net movement in any direction will be zero. Hence diffusion may be said to be movement of particles from a region of higher concentration to a region of lower concentration.

Diffusion pressure:

When more number of particles is present in a given space than what it possibly can accommodate, there will be pressure of par­ticles to migrate. This is called diffusion pressure and is proportional to the concentration of particles. In a region of less concentration there will be more space and less particles, hence there will be a deficit of pressure. This is called “Diffusion pressure deficit (DPD). In terms of diffusion pressure, diffusion pressure (lesser DPD) to a region of lesser diffusion pressure (higher DPD).


Several factors influence the rate of diffusion. There are densities, concentra­tion of the medium, temperature etc.