To diffuse means to spread, to extend in all directions, to disperse. According to the concept of molecular motion (kinetic theory), the elementary particles are in constant motion at temperatures above the absolute zero.

This results in the spread of a substance from its more abundant place to a region of less abundant place by the physical process known as diffusion. More refinery “the movement of gases, liquids or solids (molecules, atoms or ions) from a point of its higher concentration to a point of its lower concentration in order to get evenly distributed throughout the available space because of their kinetic motion is defined as diffusion”.

In the three states of matter i.e., solid, liquid and gas, the rate of diffusion of gases is faster than liquids and solids.

Examples of diffusion


The fragrance from an open bottle of perfume reaches us by the process of diffusion. The molecules of perfumes diffuse through the molecules of air. Similarly when we put sugar in a liquid, the sugar molecules diffuse throughout the liquid, giving it a uniformly sweet taste. Thus the medium of diffusion may be a gas or liquid.

Diffusion pressure

It describes the potential ability of a gas, liquid or solid to spread away (to diffuse) from an area of its greater concentration to an area of its lesser concentration the net movement of molecules results due to the development of a pressure which may be defined as diffusion pressure.

It is directly proportional to the concentration of the diffusible particles i.e., the greater is the concentration of the particles; the greater is their diffusion pressure. It is also directly proportional to the absolute temperature. Pure solvent shows maximum diffusion pressure. The diffusion pressure of water molecules in pure water is therefore greater than that of water molecules present in a sugar solution.


That is why the movement of molecules occurs from a higher diffusion pressure to a region of lower diffusion pressure.

Diffusion pressure gradient

The difference in diffusion pressures of two systems or regions constitutes the diffusion pressure gradient.

Diffusion pressure deficit


Liquids, like gases and solids have a diffusion pressure. When certain solute particles are added to the pure solvent, the diffusion pressure of the resulting solution gets decreased.

The amount by which the diffusion pressure of a solution is lower than that of its solvent at the same temperature and atmospheric pressure is known as diffusion pressure deficit. It is decreased by the addition of solutes, lowering of temperature and pressure.